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03-12-2017, 12:00 PM   #1
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understanding the 1.5 crop factor

I need help to understand a few items. My understanding is that some of the lens designed for the crop frame cameras project a smaller image circle to be compatible with the smaller sensor. If a full frame lens is mounted to a crop frame camera I assume that the image captured on the sensor is roughly 2/3 the size of full frame, thereby being the 1.5 crop factor. It seems to me then that in a sense there is a zoom effect which creates the extended focal length effect such as the 24mm-70mm being approximately equivalent to 36mm-105mm. If my assumption is correct then the 1.5 crop factor would apply to any full frame lens mounted to the APS-C cameras. Can you please tell me if I am misunderstanding all this.
Best Regards, Bob

03-12-2017, 12:12 PM   #2
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QuoteOriginally posted by jabobby Quote
I need help to understand a few items. My understanding is that some of the lens designed for the crop frame cameras project a smaller image circle to be compatible with the smaller sensor. If a full frame lens is mounted to a crop frame camera I assume that the image captured on the sensor is roughly 2/3 the size of full frame, thereby being the 1.5 crop factor. It seems to me then that in a sense there is a zoom effect which creates the extended focal length effect such as the 24mm-70mm being approximately equivalent to 36mm-105mm. If my assumption is correct then the 1.5 crop factor would apply to any full frame lens mounted to the APS-C cameras. Can you please tell me if I am misunderstanding all this.
Best Regards, Bob
It's correct as long as you understand that, were there a 24-70 designed for a crop body, it would behave identically on a crop body to a 24-70 designed for a full-frame body. It's just the focal length that matters as long as you take image circle size out of the discussion.
03-12-2017, 12:21 PM   #3
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OK, I just found the information needed for perfect explanations in older threads which were shown after I posted that question. It is only a difference with the image seen by the sensor. Thank you to all who provided the threads in the past.
Best Regards, Bob
03-12-2017, 12:21 PM   #4
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This may help in your understanding. Not only focal length but aperture for an equivalent DoF needs to be considered and multiplied by the crop factor.
Digital Camera Sensor Sizes: How it Influences Your Photography

03-12-2017, 12:23 PM   #5
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Have a read here: The Crop Factor Unmasked - Articles and Tips | PentaxForums.com
03-12-2017, 01:16 PM - 2 Likes   #6
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The whole "crop factor" thing was something invented for us old codgers who came from the film days. It gave us a frame of reference to understand what the angle of view of the focal lengths we were used to would be on digital cameras. If you're one who has never known anything other than APS-C cameras, you'll relate to focal lengths in a different way and will probably need an "expansion factor" to get an idea of how focal lengths you know will perform on a FF camera.
03-12-2017, 02:39 PM - 5 Likes   #7
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Now that you have an understanding of it, do yourself a favor. - Forget you ever heard of "crop factor".
Once you do you'll find life much simpler, and photography more enjoyable as you concentrate on learning how your lenses work on your camera(s) instead of trying to figure out how they might work on someone else's.
Unless you are actually using the same lenses on two different formats the concept is useless.
03-12-2017, 03:04 PM - 1 Like   #8
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tibbitts, thank you for the reply, slowly I am learning the wonders of this transition from film to digital.

TonyW, That is some great info, thank you.

Mark Jerling, I read all that and I am gaining a much better grasp of all this, thanks.

TomMass, I am certainly one of those old codgers and it sure is a wild ride to the new world of digital photography.

Never owned or used a APS-C camera and right now I am undecided of which format to go with. I definitely see the advantages of either. Have been considering the K3-2 or the K-1. I am sure that I would enjoy either. Most of my shots are of wildlife & landscape & with the K3 the cost sure would be far less and I could use all my old Pentax glass with either one of them. This forum is so full of fantastic advice & examples from some very talented people and that is a great help for me to learn & make decisions on equipment.

A very big thanks to all, Bob

---------- Post added 03-12-17 at 03:06 PM ----------

Parallax, that is good advice & I am going to follow it, thank you

03-12-2017, 03:18 PM   #9
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QuoteQuote:
Now that you have an understanding of it, do yourself a favor. - Forget you ever heard of "crop factor". Once you do you'll find life much simpler, and photography more enjoyable as you concentrate on learning how your lenses work on your camera(s) instead of trying to figure out how they might work on someone else's. Unless you are actually using the same lenses on two different formats the concept is useless.
Above is the best advice you will get about the 'crop factor'. I use a K3-II, and a couple of 'point and shoot' cameras. The photo I will get is whatever I see through the viewfinder. That's all you need to be concerned about: what is the camera seeing.

The only relevance of the 'crop factor' is for people who had lots of experience with 35 mm cameras -- such as me -- who were switching to digital. It helped us visualise what a particular combination of camera and lens would 'see'. I still find that helpful for my DSLR but that is because I used 35 mm cameras for a long time. It is not at all helpful for my point and shoot cameras.
03-12-2017, 03:22 PM   #10
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QuoteOriginally posted by Keith23 Quote
It helped us visualise what a particular combination of camera and lens would 'see'
Yep exactly, but now the K1s here, I can totally forget all that baloney too.
03-12-2017, 05:29 PM   #11
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QuoteOriginally posted by jabobby Quote
It seems to me then that in a sense there is a zoom effect which creates the extended focal length effect such as the 24mm-70mm being approximately equivalent to 36mm-105mm.
It is important to understand that the crop factor (versus FF/35mm frame) while giving a "field of view" equivalent to a longer lens, does not have any zoom effect from the lens itself. The subject of your picture, say a person, is rendered by the lens at exactly the same size on both aps-c and FF sensor.

When you view the resultant picture on your computer screen the aps-c image will appear larger (zoomed), but that is simply because there was a smaller field of view recorded, so everything will be enlarged more to fill the same space on your computer screen.

The K1 FF camera allows you to use FF or aps-c size. There is no difference in image quality to using the FF and cropping the image, to using the aps-c setting uncropped. Both resultant images will be produced from the same number of pixels on the sensor.

So aps-c does not give longer "reach" by itself. What will make a difference is the number of pixels/pixel density that the sensor has. For example, assuming all pixels are made equal (!) a 25mp aps-c camera will out resolve a 36mp full frame camera. This will enable you to "enlarge" (crop) your images more and still maintain image quality. But this has nothing to do with the crop factor.
03-12-2017, 06:09 PM - 1 Like   #12
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Please no laughing at my little drawing. This just shows the relationship between sensor and lens.
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Last edited by pjv; 03-13-2017 at 03:59 AM.
03-12-2017, 06:40 PM   #13
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Keith23, Now that I have a much better understanding of the crop frame concept it will greatly assist me in choosing appropriate lenses for which ever camera I decide to purchase. In fact, the lens choice will be the determining factor for which camera I get.

Kerrowdown, I agree, that almost makes the K-1 an easier choice for an old film school fella like me.

pschute, What you have stated concerning the enlargement (crop) of the image is something I have certainly noticed with many photos on this forum.

pjv, I like your little drawing, I just made one much like yours to illustrate for my nephew. But mine lacks the copyright!
03-12-2017, 06:51 PM - 5 Likes   #14
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QuoteOriginally posted by pschlute Quote
When you view the resultant picture on your computer screen the aps-c image will appear larger (zoomed), but that is simply because there was a smaller field of view recorded, so everything will be enlarged more to fill the same space on your computer screen.
I was with you until the "enlarged more" thing. Native resolution is huge on most APS-c cameras, At 100 dpi like your computer screen a K-5 image is 50 inches wide, a K-3 image is 60 inches wide, a K-1 image is 70 inches wide. In actual fact, 99.9999999% of images are reduced to a smaller size for out put. In the old days, you had to enlarge the negative more for the smaller format image to be the same size, also enlarging the grain twice as much and often creating visible grain. With digital the smaller format in this case APS-c has to be reduced less. The difference being with film, you're grain just got larger and larger. With digital the noise gets smaller and smaller and in many cases disappears completely, as you reduce an image.

The point at which a K-1 is the same as a K-3 image is probably at about 5000 pixels wide. You don't have an output device that wide. Printing at 200 dpi, you'd still have to print over 25 inches wide to see a difference, as a guess, it may not even be apparent then.

So essentially, when shooting a 24 MP full frame, and a 300mm lens ƒ2.8 , and a 24 MP APS-c with a 200mm ƒ 2.8 lens, you will have the same field of view and same resolution, in essence, identical images. Both will suffer the same amount of enlargement to print them big. That's why people say the APS-c gives you more reach. What's the difference You ask? Well it's the difference between this and this for the same image. We are talking a 3 pound total package against an 8 pound total package. One manageable one not.



This is the extreme example, showing how bad it can get. But most folks don't shoot either 200mm or 300mm so as a comparison it makes no difference. But on the whole APS-c lenses are lighter, and smaller for the same field of view.

You've got a 300mm equivalent lens for the price and weight of a 200mm with APS-c. And neither image is reduced or enlarged in size, any more than the other when viewed on a computer or printed. Once they are pixels, sensor size has nothing to do with it, (at 100 ISO, at least) in terms of enlarging or reducing.

But as Pentaxians, our FF is 36 mp, so it's a lot more complicated. You have to add Pixel Density to the equation. If there were a simple way to figure this out I'd have done it. And I haven't. But here's how much you have to crop a K-1 image before you could have taken a better image with a K-3.



The best measure of resolution for Digital Cameras is lw/ph, line width per picture height. It's the largest number of distinct lines the sensor could display if you took a picture of alternating black and white lines. It is in no way equivalent to MP. The K-1 is measured at around 3600 lw/ph tops with a great lens. A K-3 is measured at about 2700 lw/ph so the K-1 has roughly 33% more resolution, with 50% more pixels.

But a Panasonic Lumix DMZ1000 with a one inch ( 1/4 the size of APS-c) sensor can produce 2700 lw/ph just like a K-3. That little 1 inch sensor can take images equivalent to your APS-c camera at 100 ISO. (It loses lw/ph very quickly as the ISO goes up , but hang in with me, that's irrelevant.) That little sensor can produce images as good as any K-3 and better than a 22.3 MP Canon 5D mk3 FF costing 3 times as much.

So just to summarize. As has been pointed out above, the easiest thing to do, is look through the viewdifinder, take the picture you want with the camera you have, and do what you do with it. It is very unlikely that you are going to push the limits of your camera, no matter what you are shooting. And if someone is going to figure out a formula taking into account, sensor MP, real sensor resolution and sensor size more power to them, but bottom line, in digital, sensor size and MP are probably the least important things for understanding resolution or enlargeablilty.

The lw/ph is really the only thing that matters, and that exists independent of sensor size and MP, which at least IMHO opinion are at best unreliable measures of resolution.

Here's an example.


In this case, in terms of printing. the 1 inch 20 MP sensor will produce an image equivalent to a K-3 image, with a smaller file. There's a reverse magnification effect. The Lumix will give you the same resoluiton with a physically smaller image, based on how good it's sensor is at converting MP to lw/ph. The Lumix at 100 ISO will actually out resolve a Canon 5D mk 3 FF, 22 MP sensor.

You also need to take into account that different sensors in different systems lose lw/ph at different rates as the ISO increases. So, while you can might get the image you want with a smaller sensor, as the ISO climbs it will very quickly fall behind the larger sensor, as well as the Dynamic Range and noise levels.

If you don't have a headache by now tough, I do.
The only things you need to understand with crop factor is, the more crop, the shorter lens you use to achieve the same Field of View.
As general rule, the larger the sensor the better it will be in low light.
A full frame camera gives you narrower depth of field and smoother out of focus areas if you need that, and APS-c isn't good enough. (many of us find APS-c quite good for narrow depth of field images.) FF is the most overall versatile format in digital but only because it's too expensive to build faster than ƒ2 with ƒ2.8 being more common lenses in Medium Format, and 1.4 can be common in FF, and 1.2 is available. But MF will still produce the highest resolution.

Full frame is the format where it become physically prohibitive to build fast glass, for the next format up. It's in a physical sweet spot. But smaller systems can be just as useful to you if not better depending on what you do. Unfortunately landscape and wildlife are on two different ends of the spectrum. A slow snails pace camera like a K-1, D800 or 645z s great for landscape. Not so good for wildlife. K-3s, D7200s, D 750 or Canon 1DXs and many other 20-24 MP cameras are better for wildlife, with their faster frame rates and deeper buffers.

You really need one of each.

And you can know all this stuff, and still have trouble finding a camera you actually like.

Liking the camera and how it works is probably the most important thing.
Personally, APS-c is my favourite compromise, but my most used camera is the K-1, and i would buy them in that order. But I shoot a lot of small bird pictures and wildlife. If you don't or landscape is more important to you, you might go at it differently.

Aren't you glad you asked?

Last edited by normhead; 03-12-2017 at 07:22 PM.
03-12-2017, 06:53 PM   #15
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TonyW & MarkJerling, I absolutely love the beautiful information supplied on this forum. Back in the 60's, 70's & 80's a person would have to purchase several different magazines and hope to acquire the information provided here. Also it would have been the same with regards to seeing the vast number of fantastic photos that so many people post here on this forum. This forum has been as inspiration to me and I sure am excited about getting back to the pleasure of capturing the images. To all of you I say thank you again, Bob

---------- Post added 03-12-17 at 07:14 PM ----------

normhead, after reading many of the other threads regarding these issues discussed here I was expecting input from you. Very well stated and I thank you. BTW, thanks for the awesome shot of the bird. Also my nephew is planning on purchasing a Lumix 4/3 for his extended walkabouts and his reasoning is just about the same as what you stated.
Thanks again, Bob
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