[Wasp]

Another thing to keep in mind is depth of field. The physics of lenses means that a smaller image means more depth of field. The difference between full frame and crop frame is about one stop. A 200mm f/4 lens on a crop frame will be equivalent to a 300mm f/5.6 lens on full frame. Don't be confused by the extra stop, exposure does not change. In practice this means that bokeh might be slightly different from one format to the other.

More importantly, the depth of field indicator marks on an old 35mm lens are of no use on a modern APSC DSLR. Some wide angle lenses have hyperfocal distance marks, like my M 20mm and M 28mm. They show where to focus at f/8 so that everything is in focus out to infinity. For example, my trusty M 28mm has the hyperfocal distance marked as 3m at f/8. The actual value is 3.3m, but it close enough. On APSC, the hyperfocal distance moves out to 4.9m, but let's call it 5m.

For more depth of field numbers, go to:

Online Depth of Field Calculator

[mapguy]

More importantly, the depth of field indicator marks on an old 35mm lens are of no use on a modern APSC DSLR. Some wide angle lenses have hyperfocal distance marks, like my M 20mm and M 28mm. They show where to focus at f/8 so that everything is in focus out to infinity. For example, my trusty M 28mm has the hyperfocal distance marked as 3m at f/8. The actual value is 3.3m, but it close enough. On APSC, the hyperfocal distance moves out to 4.9m, but let's call it 5m.

Thanks for that, the reason for the question was how to decide on lenses with my new body. Go with old film lenses or the new DA lenses if I have a choice. I have to admit I miss the DOF guages and aperture rings.

I guess at least the DOF guages aren't of much use if your using a APSC body.

I still like the aperture rings, but even that is not as handy as the two dials now have on my k3II.

[Sandy Hancock]

Depth of field and bokeh doesn't change at all when you change formats, unless you change distance to the subject to achieve the same framing.

Unless I'm missing something fundamental, the depth of field scales should work exactly the same when you use a full frame lens on crop. Please educate me.

[mapguy]

Thanks for all the replies, I have a better handle on it and will read up on the links that were posted....

---------- Post added 03-03-18 at 07:14 AM ----------

Your calculations are correct. When I put a FF on my K3, I do not think of it as getting, let's say a 150mm lens instead of a 100mm lens. I think of it as "pre-cropping" the outermost 1/3 of the picture area .

It just dawned on me that in some ways I am loosing out....

The magnification of the lens is the same; I am just seeing less of it, which makes it look like I am closer.

I am using less of the light the lens is letting in, but probably have more sensors in a smaller area, so getting better images than using film.

Now my head hurts again :-)

I might just do the look in the view finder trick, someone mentioned. :-)

In my case all this means is I need to look for a 15mm to replace my 24mm, my old SMCA 50mm f1.4 is nice fast 75mm if i need one.

[normhead]

The magnification of the lens is the same; I am just seeing less of it, which makes it look like I am closer.

If you are using a K-3 you are seeing 24MP in the crop area instead of 15 MP, so your resolution within the crop is greater. You are in many cases seeing more detail on your subject and less of the surrounding environment. You don't get the same view with a macro lens, that you do with an electron microscope...that doesn't matter as long as you are getting the FoV that you want for the image you are taking.

There is less than a 3% difference between a 24 MP FF with 50mm lens and a 24MP APS-c lens taken with a 35mm lens. The sensor is a crop sensor, the image is not a cropped image, because you can use a wider, smaller, lighter, probably cheaper lens to do the same thing. The only thing that matters is can you get the image with the framing you want.

The difference being from the same position, you might have to use a 150mm lens on FF and a 100mm lens on APS-c, and carry a lot more weight for what many times will be exactly the same image.

I am using less of the light the lens is letting in, but probably have more sensors in a smaller area, so getting better images than using film.

The total light admitted for the same image is the same. The FF has to be one stop less to use the same ISO and Shutter speed and maintain Depth of Field. For an image with the same Depth of Field, the FF spread half the light over twice the area. If the subject doesn't have the same Depth of Field , it's a different image and you are comparing apples and orange.

---------- Post added 03-03-18 at 09:55 AM ----------

More importantly, the depth of field indicator marks on an old 35mm lens are of no use on a modern APSC DSLR. Some wide angle lenses have hyperfocal distance marks, like my M 20mm and M 28mm. They show where to focus at f/8 so that everything is in focus out to infinity. For example, my trusty M 28mm has the hyperfocal distance marked as 3m at f/8. The actual value is 3.3m, but it close enough. On APSC, the hyperfocal distance moves out to 4.9m, but let's call it 5m.

So you cheat back a little on APS-c, those markings are never all that accurate in any case. You still get a working ballpark figure. I have used the markings, and they are still reasonably accurate on APS_c. But then, most of my lenses don't even have a hyperlocal scale. I just pick a spot about 1/3 of the way out to infinity to focus on in my viewfinder and use that. IF the DoF concerns you, stop down an extra stop as a precaution.

[Not a Number]

IF the DoF concerns you, stop down an extra stop as a precaution.

There is always DOF preview too.

[normhead]

My big question is always, what good do all these things do?
There are phone apps that calculate hyperlocal distances. If you are really that concerned with doing it absolutely correctly, use one of those, not the hyperlocal scale on your lens.

[WPRESTO]

A rule of thumb I learned way back: 1) same f-stop; 2) same field-of view; 3) yields same DOF regardless of lens focal length.

Example: you exactly fill the frame, right-to-left, with an object one meter in width and set your aperture to f5.6. Whether you are using a 200mm lens or a 20mm lens the DOF will be the same because the subject-to-camera distance is profoundly different. There are on-line calculators for FOV that will show this to be true.

The idea that wide-angle lenses give more DOF and telephoto lenses less at the same F-stop is a consequence of of changing the FOV and subject distance, not because one lens gives greater DOF than the other. Photographing a small bird with a 400mm lens @ f8 and the DOF is just sufficient. But if you moved close enough with a 35mm lens to frame the bird exactly the same way, the DOF @ f8 would be exactly the same.

[normhead]

A rule of thumb I learned way back: 1) same f-stop; 2) same field-of view; 3) yields same DOF regardless of lens focal length.

Interesting but how useful is it? The difference with same FoV would seem to ignore the compression of back ground you get with a longer lens. You can have the same field of view on the subject, but not on a background with a 20mm lens and a 200mm lens. In that sense, you're not going to get the same field of view. Same size subject, vastly different background. So really not the same image. Part of FoV is the angle at which the light is gathered by the lens. A twenty mm lens is always going to drawing light in at a much wider angle that a 200mm lens.So there can be no "same field of view". Only the same field of view at a specific distance.

For so many of these things... seeing through the lens is by far the superior way to understand them. Too many explanations while interesting are of no practical use to the shooter because they are based on a limited set of parameters and assume all other things are equal tome a point. But all other things are never equal in the real world, and they may be nice facts to know, but the don't really help your photography.

[BrianR]

I'm also a fan of giving people a FF and APS-C camera to look through the viewfinder, assuming they care about these things. Old k-mount film cameras can be had for ~$20 (sometimes even functional k1000s turn up at this price), so if it's something you care about and already have an APS-C dslr, there's no real excuse not to try a practical hands-on demo.

Interesting but how useful is it?

Very. The goal of this rule of thumb is to give you a baseline for comparing some parameters of the image. It tells you how to keep the DoF and framing on your subject constant while changing the background to suit the look you want (of course the perspective on the subject changes too, as with all technical photographic concepts there's a balancing act).

[abruzzi]

I bought my first crop sensor camera a year ago. I went through the same mental calculations since I was used to 35mm. But now, having acquired a dozen primes or so, and window shopped a hundred more, my brain has internalized the new numbers like a new language. I don’t have to translate to 35mm anymore, I can think in APS-C. So while it OK to obsess over these differences now, know that soon, you will learn to just know that the DA 21 is wide enough, or maybe it isn’t and you need a DA 15 for the shot you want.

Part of the problem of all this is that 75 years ago (actually, I don’t know when) people started associating focal length with angle of view, when in fact focal length is just one variable contributing to angle of view.

[WPRESTO]

Interesting but how useful is it? The difference with same FoV would seem to ignore the compression of back ground you get with a longer lens. You can have the same field of view on the subject, but not on a background with a 20mm lens and a 200mm lens. In that sense, you're not going to get the same field of view. Same size subject, vastly different background. So really not the same image. Part of FoV is the angle at which the light is gathered by the lens. A twenty mm lens is always going to drawing light in at a much wider angle that a 200mm lens.So there can be no "same field of view". Only the same field of view at a specific distance.

For so many of these things... seeing through the lens is by far the superior way to understand them. Too many explanations while interesting are of no practical use to the shooter because they are based on a limited set of parameters and assume all other things are equal tome a point. But all other things are never equal in the real world, and they may be nice facts to know, but the don't really help your photography.

Well, what you say is true. But if you have exactly the same subject occupying exactly the same number of pixels on your sensor, and image taken at the same f-stop, then whether a 20mm lens or a 200mm lens was used the image will have the same DOF. Obviously true that, what is included in the background around the subject will be different, and the perspective on pr rendering of the subject will be different. If the subject is someone's face, straight-on, I'll wager the 200mm will please them more than the 20mm, unless it's a clown who wants an oversize nose.

[normhead]

I'll wager the 200mm will please them more than the 20mm, unless it's a clown who wants an oversize nose.

Ya, ain't it the truth.

[WPRESTO]

Interesting but how useful is it? The difference with same FoV would seem to ignore the compression of back ground you get with a longer lens. You can have the same field of view on the subject, but not on a background with a 20mm lens and a 200mm lens. In that sense, you're not going to get the same field of view. Same size subject, vastly different background. So really not the same image. Part of FoV is the angle at which the light is gathered by the lens. A twenty mm lens is always going to drawing light in at a much wider angle that a 200mm lens.So there can be no "same field of view". Only the same field of view at a specific distance.

For so many of these things... seeing through the lens is by far the superior way to understand them. Too many explanations while interesting are of no practical use to the shooter because they are based on a limited set of parameters and assume all other things are equal tome a point. But all other things are never equal in the real world, and they may be nice facts to know, but the don't really help your photography.

Well, what you say is true. But if you have exactly the same subject occupying exactly the same number of pixels on your sensor, then an image taken at the same f-stop, whether a 20mm lens or a 200mm lens was used, will have the same DOF. Obviously true that, what is included in the background around the subject will be different, and the perspective on or rendering of the subject will be different. If the subject is someone's face, straight-on, I'll wager the 200mm will please them more than the 20mm, unless it's a clown who wants an oversize nose.

[normhead]

I bought my first crop sensor camera a year ago. I went through the same mental calculations since I was used to 35mm. But now, having acquired a dozen primes or so, and window shopped a hundred more, my brain has internalized the new numbers like a new language. I don’t have to translate to 35mm anymore, I can think in APS-C. So while it OK to obsess over these differences now, know that soon, you will learn to just know that the DA 21 is wide enough, or maybe it isn’t and you need a DA 15 for the shot you want.

Part of the problem of all this is that 75 years ago (actually, I don’t know when) people started associating focal length with angle of view, when in fact focal length is just one variable contributing to angle of view.

I was in school 50 years ago, and we used 8x10 or 4x5 film for most of our assignments. We knew the simple 300mm portrait lens we used in the studio on the view cameras was very different on a 35mm camera.

Funny no one ever called 5x7 or 4x5 film crop cameras. That's a modern notion. A camera is camera. Period, end of discussion.

What is different now is there are a lot of incorrectly expressed opinions out there published by people who aren't trained in photography, printing half truths and over simplifications that don't really do justice to a real understanding of the subject.

The difference between those with training at a post secondary institutions ind internet bloggers? When they are wrong, in a formal course, they fail. If you are wrong on the internet you can keep blabbing on for years.

The goal of this rule of thumb is to give you a baseline for comparing some parameters of the image.

It's not as good as looking through a viewfinder, where every parameter is correctly interpreted and displayed as it will look when you are shooting. Almost every one of these mathematical simplifications don't tell the whole story.

This is the endless debate between those who see photography as a mathematical discipline, and those who think experiential opinion based on use of the lens. In my experience, you select your lenses based on the huge undefinable understanding of how the lens renders, which will never be completely expressed in mathematical formulas. There's no way to get away from it. You have to learn the lens, not the numbers. The numbers tell you next to nothing. The part about the numbers you can understand in minutes, understanding how to make best use of the lens and what it can do for you may take months.