[pentaxk3user]

Thank all of you for taking the time to help me out .Looks like a got a lot of reading to do. I like to know how and why things work . The first thing I think by reading everyone's post is that I need to read about dof and find out exactly what it is . I thought dof was once you have something in focus in the lens , its how close behind the thing in focus is out of focus, so the more the lens is open the shorter the distance behind what is in focus is out of focus. I didn't know that just because the sensor was smaller it had a different dof. I thought the dof would stay the same and the only difference between the full frame and the smaller sensor was the size it captured. for example if you stood up a tape measure lets say it would show in the photo from the ground to the top 10 foot but on the smaller sensor you would only get just guessing say 6 foot on the tape measure . I thought the crop factor was the only thing that changed the total size of the photo. Why would the dof change just because there is less back ground from the center to the edge .why would the dof be a f4 value like Howieb 101 post above . Why does the total size captured change the dof . If lets say for example and I know the number are not going to be correct , but say close to the center of both full and smaller sensors you focused on something 5 foot away and just behind what was in focus lets say you put a object 2 foot behind and just to the right of what was in focus so you captured both objects in both full frame and k3 sensor . wouldn't that object in the back ground be out of focus exactly the same amount on both . If you stop the video at 6:44 when he is moving from the head to the corner at 6:44 you can see the object the book shelf I think it is ,It looks like the 1.8 is sharper on the book case than the 2.8 .the 2.8 looks out of focus more I'm talking about whats just to the left of the face . I think this is whats getting me so confused. I thought the 1.8 would be more blurry but its not . the object is the same size, its the same distance behind the head ,the head is in focus on both photos ,the cameras are both in the same place yet the 2.8 is making the object blurrier in the back ground compared to 1.8 that I would think would make it blurrier not sharper . I think once I under stand why this is happening things will make more sense to me .

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Last edited by pentaxk3user; 05-02-2015 at 11:35 AM.

[UncleVanya]

Exposure is not measured as the sum total of all light on the frame - it is measured based on average light over the frame per unit. If you shoot a boring but uniform patch of green grass, and you start in FF mode on a Nikon or Canon and then switch to cropped mode - the exposure stays the same. The camera correctly ignores the light outside the cropped area since it isn't the total amount of light - but the light per unit of area that matters. In more complex scenes switching from FF to crop may impact the exposure due to changes in the proportion of light and dark objects left in the picture given the change in field of view - but that is not due to "losing" light, it's just the same as though you reframed the picture - the exposure would change either way due to what the subject matter was not due to the sensor size.

This is a simple way to prove to yourself that "light" lost by not hitting the sensor is not relevant to measuring how fast the lens is. The same applies if you use a cropped lens or a FF lens - the cropped lens creates a smaller image circle that doesn't light up the FF area. Using this on a FF camera without cropping will cause exposure issues since the camera will see the unlight area of the sensor as "black" and factor that into the exposure. This will then OVER EXPOSE the section of the picture that is covered because the meter will have tricked itself. Manually setting the exposure to the correct value will show this is a metering error not an actual loss of light.

Does this impact Depth of Field in a practical way? Maybe but I really don't care as I can typically get the type of image I want without crazy shallow depth of field anyway.

In the final analysis... EXPOSURE IS UNCHANGED BY THE SENSOR SIZE - even metering is going to be close as long as the lens covers the entire image circle of the sensor selected. For a given ISO and f/stop you use the same shutter speed (baring minor lens design light loss differences). Yes some sensors do worse at higher ISO so this assumes we are starting at an ISO both sensor sizes can handle easily.

---------- Post added 05-02-15 at 02:41 PM ----------

If you take the same lens at the same focal length and distance the DoF is the same. The field of view and reframing the image to get the same picture is what drives DoF differences. So your picture at 100mm on FF and APSC will look different shot at 15' but they will both have the same depth of field in that case.

At least that is how I understand things. The trouble is most of the time you don't do that. You reframe the shot and change the focal length used or you change your subject distance.

[nozoom]

I thought the dof would stay the same and the only difference between the full frame and the smaller sensor was the size it captured

You got it right. This is correct. Look at the attached image. It consists of two images, the one with the red border and the one with the yellow border.

The image with the red border is taken with an old Konica-Minolta Dimage A2 which has a pretty small sensor (8.5 mm x 6.6 mm). It has a zoom lens, and I have adjusted it to as near 35 mm as I could (now, that is 35 real millimeters, no crop quasi focal millimeters).

The image with the yellow border is taken with at Pentax K3 with at 35 mm lens (still real millimeters, and sensor size is 23.5 mm x 15.6 mm).

Both images are taken from the same standpoint and with the same aperture (f/4.0).

So: same distance, same focal length and same aperture, but different sensor size.

Since the Dimage A2 has a much smaller sensor than the K3, it will cover a proportionally smaller part of the K3,s image. If the K3 sensor had been just as large as the A2's sensor it would have taken the green bordered image. But since the sensor was not that small it took the yellow bordered image.


Now, the crop factor between those two sensors are huge: 2.67! How has this affected the DOF? See for yourself.

[pentaxk3user]

Exposure is not measured as the sum total of all light on the frame - it is measured based on average light over the frame per unit. If you shoot a boring but uniform patch of green grass, and you start in FF mode on a Nikon or Canon and then switch to cropped mode - the exposure stays the same. The camera correctly ignores the light outside the cropped area since it isn't the total amount of light - but the light per unit of area that matters. In more complex scenes switching from FF to crop may impact the exposure due to changes in the proportion of light and dark objects left in the picture given the change in field of view - but that is not due to "losing" light, it's just the same as though you reframed the picture - the exposure would change either way due to what the subject matter was not due to the sensor size.

This is a simple way to prove to yourself that "light" lost by not hitting the sensor is not relevant to measuring how fast the lens is. The same applies if you use a cropped lens or a FF lens - the cropped lens creates a smaller image circle that doesn't light up the FF area. Using this on a FF camera without cropping will cause exposure issues since the camera will see the unlight area of the sensor as "black" and factor that into the exposure. This will then OVER EXPOSE the section of the picture that is covered because the meter will have tricked itself. Manually setting the exposure to the correct value will show this is a metering error not an actual loss of light.

Does this impact Depth of Field in a practical way? Maybe but I really don't care as I can typically get the type of image I want without crazy shallow depth of field anyway.

In the final analysis... EXPOSURE IS UNCHANGED BY THE SENSOR SIZE - even metering is going to be close as long as the lens covers the entire image circle of the sensor selected. For a given ISO and f/stop you use the same shutter speed (baring minor lens design light loss differences). Yes some sensors do worse at higher ISO so this assumes we are starting at an ISO both sensor sizes can handle easily.

---------- Post added 05-02-15 at 02:41 PM ----------

If you take the same lens at the same focal length and distance the DoF is the same. The field of view and reframing the image to get the same picture is what drives DoF differences. So your picture at 100mm on FF and APSC will look different shot at 15' but they will both have the same depth of field in that case.

At least that is how I understand things. The trouble is most of the time you don't do that. You reframe the shot and change the focal length used or you change your subject distance.

How do you get less dof from a ff camera set at the same 35mm setting they both are set to 35mm. He said at 6:30 that the ff lens has less dof . how can you have less dof at 2.8 compared to 1.8 on smaller sensor . what makes it possible that a aperture closed more on one sensor has less dof that a lens thats open more 1.8 but comes out sharper in the background . He is not cropping the smaller sensor ,I would think he had to cut off extra on the ff to make it the same size as the smaller sensor . So every thing being the same except the aperture setting why does the ff photo have more blur in the back ground . If someone answers it will you dumb it down a little for me ,I'm not getting a lot of what you good kind people are saying its going way over my head right now . I hope some day to under stand it but I'm not there yet .

[Sandy Hancock]

If you put the same lens at the same aperture on a full frame and APS-C camera, you will need to get closer to the subject with the full frame to have the subject the same size. As such, the depth of field in the resultant image will be shallower.

If you take both shots from the spot the depth of field will be identical, but the subject will be smaller on the full frame.

[pentaxk3user]

You got it right. This is correct. Look at the attached image. It consists of two images, the one with the red border and the one with the yellow border.

The image with the red border is taken with an old Konica-Minolta Dimage A2 which has a pretty small sensor (8.5 mm x 6.6 mm). It has a zoom lens, and I have adjusted it to as near 35 mm as I could (now, that is 35 real millimeters, no crop quasi focal millimeters).

The image with the yellow border is taken with at Pentax K3 with at 35 mm lens (still real millimeters, and sensor size is 23.5 mm x 15.6 mm).

Both images are taken from the same standpoint and with the same aperture (f/4.0).

So: same distance, same focal length and same aperture, but different sensor size.

Since the Dimage A2 has a much smaller sensor than the K3, it will cover a proportionally smaller part of the K3,s image. If the K3 sensor had been just as large as the A2's sensor it would have taken the green bordered image. But since the sensor was not that small it took the yellow bordered image.


Now, the crop factor between those two sensors are huge: 2.67! How has this affected the DOF? See for yourself.

I don't see a big change in dof between the 2 they look to be blurred about the same . So why is he saying in the video you have a shallower dof with a ff camera . It seem you would get the same dof just as smaller portion of that a ff camera would . So I should be able to get the same dof on my k3 as a ff camera with the same mm lens it will just be a smaller portion of the image compared to the ff camera .

---------- Post added 05-02-15 at 03:22 PM ----------

My wife is giving me a green light on the sigma 18-35. I use my kit lens a lot at 18 to 35 more than any other distances and if I want something close I use my 70-200 but i'm not happy at all with the 18-50 kit lens Its the one that came with my k10d a long time ago . Its just not very sharp no matter what I do . So I thought I would try the 18-35 to replace the kit lens. I want thing to be out of focus behind at 1.8 but sharp on object or person I have in focus . and this looks like its the best and sharpest lens I can find thats between 18- and 35 mm that zoom and that has a 1.8 . So I was going to hurry up and order it before she changes her mind. I guess once I get it I will figure out the dof thing and how it works on this lens and my camera at 1.8 .

[normhead]

That debate is long, and this guy, Tony Northrop has a point, because on an APS-C, a 200mm lens will cover the same image surface as a 300mm lens, and some of the light received and transmitted by the lens will not hit the sensor.

Is this enough to say that a 200mm F2.8 is equal to a 300mm F4.2 on APS-C cameras, only a real test where the exposure time is measured can tell. But from what I have seem until now, it is so.

Anyway, a Sigma DG 70-200mm f2.8 is a good lens. Enjoy it.

We,ve done this so many times.......

FIrst, in terms of exposure time ƒ2.8 is ƒ2.8 not ƒ4.2.

IN total light terms.... to get the same DOF using an FF camera, you have to stop down one stop on the FF camera to get the same DoF. To use the same exposure time, you'd have to double your ISO. Because you have half the light coming through the lens, the light intensity on the APS-c sensor is twice as much. So the total light absorb is the same. Northrup is an idiot. He didn't think it through. He and every other FF propagandist that follow this line of thought, seem to forget, they have to stop the FF sensor down, to get the same DoF.

And because you've stopped down and doubled your ISO, the noise will be the same as well.

If I have to explain this one more time, I'll probably pull my hair out. That's not as bad as it sounds, I don't have much hair.

[dakight]

We,ve done this so many times.......

FIrst, in terms of exposure time ƒ2.8 is ƒ2.8 not ƒ4.2.

IN total light terms.... to get the same DOF using an FF camera, you have to stop down one stop on the FF camera to get the same DoF. To use the same exposure time, you'd have to double your ISO. Because you have half the light coming through the lens, the light intensity on the APS-c sensor is twice as much. So the total light absorb is the same. Northrup is an idiot. He didn't think it through. He and every other FF propagandist that follow this line of thought, seem to forget, they have to stop the FF sensor down, to get the same DoF.

And because you've stopped down and doubled your ISO, the noise will be the same as well.

If I have to explain this one more time, I'll probably pull my hair out. That's not as bad as it sounds, I don't have much hair.

You're being much kinder to them than you were to me...

[normhead]

You're being much kinder to them than you were to me...

It's become so common place now, it's not a big deal. The outrage I felt when I realized folks like jsherman had been hoodwinking me for years has subsided. Actually it's so simple and concise I can't believe it took me so long to figure it out. I knew the way equivalence was being used was wrong, right from the start, I just couldn't explain why. I even knew what part of how they were explaining the equation was wrong. It still took me a year and a half to clue into what exactly they were doing.

To make these arguments, you simply keep everything else equivalent but the DoF. You simply say, "with the FF you just stop down" ignoring that doing so changes the ISO. Change you have to make to maintain equivalence, which wipes out all the FF advantage.

SO now we're back to you have to go FF if you want more resolution than APS-c can provide, or if you want features that aren't offered on APS_c cameras. I believe the Nikon D750 has a faster burst rate and much faster autofocus, and those are not small things...But those are things you have to look at camera by camera. The 6D, D610 and D810 have slower burst rates. You have to shop features, not sensor size.

So now it's pretty much old news. Nothing fuels outrage more than realizing you been had.

[reeftool]

So now it's pretty much old news. Nothing fuels outrage more than realizing you been had.

That's modern marketing. In cameras, it's built around BS specifications they want us to believe are important. For 90% of my shots, my K10D did the job just as well as the K5 and you would have to look at the exif to tell the difference. In camera and lens marketing, equivalence refers to the field of view and magnification. Back in the film days, to learn photography, you bought a book or borrowed one from the library that explained all the different things about exposure, DOF, etc. They were quite thick and the information in them was accurate because the publishing houses had editors that checked them over. Today, people buy DSLR's and never open the manual but go watch a 5 minute YouTube video from some clown who has been shooting for 6 months and now knows everything.

[VoiceOfReason]

I cheated and read Understanding Exposure after falling victim to focal length equivalence and thinking that a 50mm lens made for FF wouldn't be the same on APSC as a 50mm made for that format. I am glad that early on someone nailed me with that, educated me on it, and recommended that book.

[pentaxk3user]

If you put the same lens at the same aperture on a full frame and APS-C camera, you will need to get closer to the subject with the full frame to have the subject the same size. As such, the depth of field in the resultant image will be shallower.

If you take both shots from the spot the depth of field will be identical, but the subject will be smaller on the full frame.

I'm sorry I watched the video again and I finally read at the bottom I thought was a ad, he said he put the ff on 70mm not 35 to make thing the same angle of view . So would that 70mm lens make a different dof compared to the 35mm . Is that why the back ground with the ff has more blur and nothing to do with the fact its ff . What are some of the best books out there to read that explains different mm lens and the effect it has on dof and just some good books that explain how aperture setting and the effect it has on things . I'm going to get Understanding Exposure the one VoiceOfReason mentioned . I think its time I start getting educated . I think knowing what all of you know already will help me take better photos .

[nozoom]

books out there to read that explains different mm lens and the effect it has on dof

DOF is a very theoretic, subjective and also diffuse and "woolly" concept. It depends on what you select to be the smallest detail in the image that you define to be a single pin sharp point (well, maybe not pin sharp, but sharp enough to be a point), the so called circle of confusion (CoC). The CoC number is the diameter of a circle that resembles such a point. The problem is that in practical circumstances the size of such a point will depend on how big your final print is and from what distance you view it. Have you been looking at a painting from very near? It will (normally) be very unsharp and blurry. Then you move backwards, and you see it much sharper. This is the same with DOF. When you move backwards from your image image (your print of it on the wall), the DOF will inevitably increase. What was unsharp from short distance will look sharp from further away. If you, instead of moving backwards stand on the same position, but make a much smaller sized print, then it will also look sharper. The DOF has increased. This is very easy to demonstrate for yourself. So when you take the image, what size of the print shall you select your aperture for so you get the correct number of inches of DOF? And also from what distance shall the image be viewed? The DOF calculator (or your calculation if you learn how to do it, its not that difficult either) might be saying that your depth of field is 50 cm. But when you view your 3 ft x 2 ft print on the wall from 1 ft away, you will most certainly not see 50 cm of depth of field. Neither will it be 50 cm if you look at a 6 in x 4 in image from 3 ft away. So how useful are such numbers? In my point of view they are of very limited use. I don't take images for exact print size and viewing distances, so how can they be useful? Instead of calculating such numbers I believe it is much more important to experiment with the gear you have and see the results. Take images with varying apertures. Scrutinize the images. How did the aperture influence the image? Gain experience. Reading books might gain valuable insight, that's for sure, but must not come instead of experimenting yourself. In my point of view. ;-)

Cheers,

(and excuse my English, it's not my native language)


Btw: There is an article about DOF on Wikipedia: http://en.wikipedia.org/wiki/Depth_of_field

[dakight]

Understand that for practical purposes, the difference between a FF and an APS-C camera is the size of the sensor. The only difference between the images taken with the same lens of the same object at the same distance from that object will be the amount of that distant scene that will be captured and recorded. An APS-C sensor will be exposed to a segment of that distant scent while the FF will be exposed to a segment approximately 1 1/2 times larger. That is the FF will have a wider field of view. Everything else about the image will be identical, the exposure settings, the depth of field, the color balance, the noise, all will be the same assuming the only difference is sensor size.


Equivalence is an attempt to reconcile the difference in size of the image captured by different size sensors. WHere depth of field gets confused is that to obtain the same field of view on a FF sensor as is seen on an APS-C the observer must move toward the object to make it occupy the same proportion of the frame as the same object does on an APS-C Sensor. If everything else remains the same, simply moving closer to the object of focus will shorten the depth of field which means that the supposed equivalence does not exist, to compensate one must stop down to increase the depth of field to something approximating that of the APS-C image. But stopping down changes the exposure so one must then increase ISO which in turn leads to an increase in noise and a potential loss of Image Quality. Equivalence is a self-defeating scheme to make one thing look like something else. Give it up, throw it out, forget about it. Snap the lens on your camera and go shoot pictures. You will soon learn what works and what doesn't, you will be a more competent and confidant photographer.

[pentaxk3user]

Wow this is great information about dof , ff vs aps-c , aperture . As I was doing my Pentex forum search I could not find any of this and I typed a bunch of stuff .I should change the name of the post so other people like me that don't under stand every thing yet could read what you guys and girls have typed . I hope you all don't mind if I ask another stupid question but here it goes , If the 18-35 is made for aps-c sensors and never meant to go on a ff camera ,really a 18-35 mm .lens If its not why doesn't it say what it is .Why do I keep hearing you have to multiply things to get what it really is . like a 18-35 mm is a 29-56 mm or what ever it works out to be. Why don't they just put what it is on the lens if it only goes on the aps-c camera.

---------- Post added 05-03-15 at 07:53 PM ----------

I found this video

Youtu.be

Youtu.be

I like using my 70-200 2.8 for birds at 200mm , and he said it was good to put this kind of lens on a aps-c sensor and be sharper than on a ff camera . So thats good news for me .