[pentaxk3user]

Is a sigma dg lens that's 2.8 really 2.8 on my k3. I watched a video and this guy said that any lens that's not made for the aps-c would not be 2.8 . but it was 2.8 time 1.6 or some number like that . So what would my 2.8 sigma dg 70-200 lens be if its not 2.8 . What is the number I have to times by to get the right f stop number.

[VoiceOfReason]

Aperture is a function of the opening on the lens and focal length. f/2.8 will be f/2.8 regardless of format. Also, the guy shoots Canon with that 1.6 crop factor, so what do you expect? He is like an uninformed shooter!

[FantasticMrFox]

Is a sigma dg lens that's 2.8 really 2.8 on my k3. I watched a video and this guy said that any lens that's not made for the aps-c would not be 2.8 . but it was 2.8 time 1.6 or some number like that . So what would my 2.8 sigma dg 70-200 lens be if its not 2.8 . What is the number I have to times by to get the right f stop number.

You're mixing up a bit of stuff here. The aperture is physically the same, no matter what format you use, and it also lets in the same amount of light. The one thing that changes with format is the equivalent depth-of-field (DOF), which will be shallower on a larger format (e.g. FF) as compared to a smaller format (e.g. APS-C) for the same aperture value.

And the crop factor that you have to multiply the aperture with for this purpose is 1.5 for APS-C (including your K3), it's only 1.6 for Canon APS-C cameras.

[Na Horuk]

Is a sigma dg lens that's 2.8 really 2.8 on my k3. I watched a video and this guy said that any lens that's not made for the aps-c would not be 2.8 . but it was 2.8 time 1.6 or some number like that . So what would my 2.8 sigma dg 70-200 lens be if its not 2.8 .

I would just ignore all of that if I were you. Aperture is a lens property, independent of camera. f2.8 will give you a certain brightness (t-stop) regardless of where you mount the lens (35mm film, FF, crop sensor, medium format). This is what is important.
Also, any f2.8 lens that is "made for APSC" will still give the same f2.8 as a lens "made for FF". This means that they will give the same brightness and same DoF at the same focal length, focus distance, and scene illumination.

Basically there is a whole debate about "equivalence", but it is rather silly and has practically no application in real life. Best to go on with your life and enjoy your gear. That is a nice telephoto you have, it will serve you well.

Edit:

which will be shallower on a larger format (e.g. FF) as compared to a smaller format (e.g. APS-C) for the same aperture value.

Nope, depth of field depends on the lens aperture, which is independent of sensor size. Only way the sensor could play a role in this is that the sensor resolution/CoC might affect the perception of DoF in the final image, but even that is not strictly dependent on sensor size (though, smaller senors tend to have higher resolution).
Depth of field will only be wider on APSC when you use the same focal length and aperture, but move farther away from the subject to keep the same framing as you would have with an FF. But that is an odd way to go about doing things.

Ugh, sorry about this addition to my post. This is where the silly "equivalence" debate starts. And it doesn't seem to end. Plenty of heated threads about this, if you are interested. But don't let it confuse you.

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Last edited by Na Horuk; 05-01-2015 at 06:55 AM.

[Dartmoor Dave]

Yes, f/2.8 is f/2.8 no matter whether it's on APS-C, "full frame" digital, medium format, or the Hubble telescope. Soon a variety of fanatic called "equivalentists" will show up and try to convince you otherwise, because of their belief that all lenses in all formats should be compared to the depth of field of digital "full frame".

[marcusBMG]

You've been watching a video by smoothie Tony Northrup haven't you? That guy causes a lot of trouble!!

The answer to your question is yes AND no.
The most important part of that is NO. When we think about f stop we are first thinking about exposure. The exposure depends upon light intensity, and light intensity arriving at the sensor is determined by the ambient light and then the lens f-stop. The f-stop is a physical property of the lens and the size of the sensor is irrelevant. If you have an f2.8 lens on eg the Q5 with a tiny sensor (5x crop factor), and on a full frame camera, both will be using the same shutter speed for the same exposure (other things eg asa being equal).
However what is affected is the depth of field. To get the same image in terms of field of view with those two cameras you will need a lens with one fifth the focal length on the q5 compared to the full frame. however the images will not be identical. The depth of field will be much greater with the lens on the q5, rather obviously because it will be a much shorter focal length eg 60mm instead of 300mm (300/5=60). To get the comparison quantified, multiply the f-stop by the crop factor. Thats the yes part of the account.

But the final thing to say is: don't worry about it. Just familiarise yourself with the image results on your camera and use lenses and f-stops accordingly. It's only something to think about if you are using different format cameras a lot.

[normhead]

Lot's of good advice here, definitely better than the video you watched.

[UncleVanya]

Using your f/2.8 lens you will get the same shutter speed and the same exposure at the same ISO (roughly) as anyone else using any other f/2.8 lens.

I'm not going to get into the depth of field discussion which is a bit muddier - suffice it to say, if you like the way the photos look - who cares.

[Sagitta]

I like to pull this video out for a visual guide to understanding crop factor.

Youtu.be

Youtu.be

The truck is the image cast by the lens upon the sensor. The bridge is the undersized sensor, and thus crops the image. A 35mm is a 35mm no matter what camera its mounted on. If you could hotglue it to an iPhone it would still be a 35mm lens, the smaller sensor simply crops the edges off the frame leaving you whatever is in the center.

[boriscleto]

I like to pull this video out for a visual guide to understanding crop factor.

https://youtu.be/xzkWTcDZFH0

The truck is the image cast by the lens upon the sensor. The bridge is the undersized sensor, and thus crops the image. A 35mm is a 35mm no matter what camera its mounted on. If you could hotglue it to an iPhone it would still be a 35mm lens, the smaller sensor simply crops the edges off the frame leaving you whatever is in the center.

We have several low RR bridges in Syracuse that get hit all the time. One just had the roadway lowered but the other is on the lake shore so lowering the roadway would put it below the water table. That bridge has no fewer than 12 low bridge signs spread over several miles of road but it still gets hit. A MegaBus double decker hit it and a passenger was killed. The driver missed his exit and was illegally following a GPS.

[JimmyDranox]

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.

[dakight]

It's easy to get confused by "equivalence," I know, I've been there. The best thing to do is forget about it, learn the characteristics of your equipment and enjoy it. About the only thing you can reliably determine using equivalence is field of view and you can see that through your viewfinder anyway.

---------- Post added 05-01-15 at 09:11 AM ----------

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.

No, he doesn't have a point. F-stop is the ratio of entrance aperture diameter to focal length of the lens; it depends entire on the physical characteristics of the lens irrespective of what camera it may be mounted on. The f-stop is the f-stop, period, end of story. I made that same mistake sometime back and realized where I had gone wrong when I did the homework to understand what aperture is.

[JimmyDranox]

No, he doesn't have a point. F-stop is the ratio of entrance aperture diameter to focal length of the lens; it depends entire on the physical characteristics of the lens irrespective of what camera it may be mounted on. The f-stop is the f-stop, period, end of story. I made that same mistake sometime back and realized where I had gone wrong when I did the homework to understand what aperture is.

This can be debated for a long time. but from what I know not only from photo, but also from astronomical optic, I think is right. The intensity of the light received is depending not only by the lens size, but also by the receiver size, whatever is eye or sensor. And in astronomy we have special formulas to match the parameters of the system, even for visual observations, where the size of the maximum eye pupil at different ages varies. And we must choose the right eyepiece for a certain telescope, and certain type of observation, and our age.

[Sagitta]

What differs is the size of the medium the image is projected on. You're going to lose some of that image on the cropped sensor, but the light itself isn't changing at all, since it will always be the same distance from the medium.

Visual example: Small cap is APC-C, big cap is FF. Each cap is getting hit by the same amount of light (OK, the Samyang cap may be vignetting a hair, but you get the idea...). The amount per bit of space on each cap is exactly the same. No lens is going to behave any differently for one type of medium as opposed to another - it will still allow the exact same amount of light through depending on how you have it set up.

[dakight]

Ignoring lens flaws such as vignetting, the light density, that is the amount of light to which each pixel is exposed, is identical regardless of the sensor size. Exposure depends on light density, not on total light which is an entirely different concept and argument. To be sure, there are factors which affect image quality, some in favor of the smaller sensor, others detrimental to it but the light density, on which exposure is determined is subject only to the characteristics of the lens and not dependent on the camera or sensor at all. A lens with a 50mm focal length and a 25mm front pupil diameter will be an f/2.0 whether it is mounted to a compact camera with a tiny sensor or an 8x10 format camera or anything in between or even beyond.