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05-11-2017, 08:52 AM   #1
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f-stop crop

Sorry if this is common knowledge, but I wasn't really able to find a reference on this, so I figured I'd ask:

So I now have a K-3 II with a 18-135. I understand the smaller APS-C sensor size on field of view compared to a 35mm film camera or a full frame DSLR. But I'm trying to understand the effect on f-stops.

As I undersand it, the F number is just the ratio of the focal length of the lens to the diameter of the aperture. So a 50mm lens with a 25mm maximum aperture is F/2.

Now, lets say I have an APS-C 50mm F/2 DA lens. It is designed to project a circle of light on the sensor that large enough to cover the entire sensor (but not large enough to cover a frame of 35mm film or a full frame sensor.)

Now if I compare this lens with a full frame 50mm F/2 DFA lens. The focal length is the same the aperture is the same, but the image circle it projects is larger. This is where my first question comes in. Since that image circle is larger, but the aperture is the same, can I assume that the FF lens is slower than the APS-C lens at the same F-stop because the same amount of light is being spread over a larger area?

In addition, will the depth of field will change, and in what direction?

The reason this is entering my head is this: I'm thinking if I want to get a prime, the size I want has limited options. So I might be looking at a new APS-C lens and comparing it with an older film lens, and I want to compare like to like.

05-11-2017, 09:11 AM   #2
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Short answer is no, the FF lens is not slower. An FF lens has a wider field of view than it's APS-C version which means it is collecting proportionally more total light to cover that larger image circle.

Aperture and image circle are totally independent of each other although it is optically challenging to design and build lenses of given focal length (especially wide angle lenses) that have both big apertures and big image circles.

Another way to think of it is that the focal length and aperture are physical properties of the lens and the lens' properties (e.g., brightness & depth of field) don't change just because you've mounted it on a camera with a bigger or smaller sensor. The depth of field of a new APS-C lens on an APS-C camera will be identical to the depth of field of an old full-frame film lens (of the same focal length and aperture) on an APS-C camera.

Where things get messy is in comparing depth of field across formats (APS-C vs. FF) because creating the same picture on two different cameras involves using different focal lengths, different optical magnifications, and different blur circle diameters.
05-11-2017, 10:06 AM   #3
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QuoteOriginally posted by photoptimist Quote
Short answer is no, the FF lens is not slower. An FF lens has a wider field of view than it's APS-C version which means it is collecting proportionally more total light to cover that larger image circle.

Aperture and image circle are totally independent of each other although it is optically challenging to design and build lenses of given focal length (especially wide angle lenses) that have both big apertures and big image circles.

Another way to think of it is that the focal length and aperture are physical properties of the lens and the lens' properties (e.g., brightness & depth of field) don't change just because you've mounted it on a camera with a bigger or smaller sensor. The depth of field of a new APS-C lens on an APS-C camera will be identical to the depth of field of an old full-frame film lens (of the same focal length and aperture) on an APS-C camera.

Where things get messy is in comparing depth of field across formats (APS-C vs. FF) because creating the same picture on two different cameras involves using different focal lengths, different optical magnifications, and different blur circle diameters.
Does that also mean that T-stops aren't affected?
05-11-2017, 10:25 AM   #4
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QuoteOriginally posted by timw4mail Quote
Does that also mean that T-stops aren't affected?
The T-stop for a given lens should be the same whether mounted on an APS-C or FF camera.

But two different lenses of the same f-stop may have different T-stops because the T-stop depends on the number of elements in the lens and the quality of the lens coatings. Old film lenses tend to have fewer elements (good) but worse coatings (bad).

05-11-2017, 10:38 AM   #5
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Thanks. I was getting a bit down the rabbit hole on my thinking there. I'm not worried about calculating conversion between full frame sensors and APS-C. Seems easier, since I have an APS-C camera to just learn the numbers in APS-C world and internalize them instead of trying to always think in 35mm terms. I want to find a prime around 24mm since it gives the field of view I always liked, but there doesn't seem to be much.
05-11-2017, 02:41 PM   #6
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The larger FF sensor has the advantage of providing shallower depth of field. A larger sensor also captures more light. The APS-C sensor has a crop factor (smaller image area). Trying to create a similar image (FOV and DOF) from the same shooting position with the two different formats is an interesting exercise. Shooting with a K-3 and a Sigma 18-35 set at 33mm and f/1.8 will provide a similar field of view to a K-1 with a 50mm focal length and f/2.8 aperture. Appling the crop factor to both the focal length and aperture will help you visualize the differences between formats.
05-11-2017, 03:31 PM   #7
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The depth of field is actually the same, but the DoF you can achieve differs. The way to think of it is this:
Put a 50mm on a FF body and shoot a head and shoulders shot at f2.8 and you'll get nice shallow DoF.
Put that same lens on an APSC body and, to compose the same shot, you will need to stand further away due to the crop. That increased subject distance increases the DoF.
However, if you keep the same subject distance, the DoF will be the same...but you would see less of the subject.
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