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Forum: Troubleshooting and Beginner Help 05-11-2017, 10:38 AM  
f-stop crop
Posted By abruzzi
Replies: 6
Views: 836
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.
Forum: Troubleshooting and Beginner Help 05-11-2017, 08:52 AM  
f-stop crop
Posted By abruzzi
Replies: 6
Views: 836
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.
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