Math

Hey guys

I have a few things which makes me think, when it comes to logical thoughts on lenses etc.

If i have a 50mm 1.4 lens on FF, will it give me the same depth of field like a 100mm 2.0 lens on FF?

My thesis: as focal length is a factor for the DOF, it would be logical when you double the amount of light coming through the lens, you would need half the focal length required to get the same DOF and bokeh (despite that every lens renders it different)

Is that true?

Then, why does fullframe sensors offer a better isolation with the same lens on crop? Cause, lets say you have a 50mm 1.4 lens: On FF thats effective 50mm FOV with great isolation at 1.4. If you put it on a crop sensor, the FOV becomes something around 80mm equivalent on FF. So the DOF and isolation is more as the same lens at same aperture. So if crop sensors has less DOF than same lenses on FF, it should be almost the same. But why isnt it like that?

Here ya go..

http://www.dofmaster.c​om/doftable.html

50d vs 5DMIII

So lets go with 10 feet:

50D
50mm 9' 8.3'' 10' 4'' is the focus range, or about 8 inches.
100mm 9' 11.1'' 10' 1'' or about 1.9 inches.

5DMIII
50mm 9' 6.2'' 10' 6'' or about 12 inches.
100mm 9' 10.5'' 10' 2'' or about 1.5 inches.

Not sure if this supports or kills your theory... this is comparing by distance, and I believe a more accurate comparison is the true field of view, standing where the crop camera has a 'full' picture, and then moving the full frame camera to a distance that will give the same field of view /crop of a picture will alter the distance, and therefore the range of items in focus. So to take the same picture, with the same in camera crop requires that the full frame camera is closer to the subject.. Yes?

But I can tell you that Bokeh is also very dependent on how many leave's (leaf's?) there are in a lens, the fewer leaves to stop down with, the harsher the bokeh. And why many prefer more expensive lenses they have better bokeh on either a crop or a FF.

davidfarina wrote in post #16069477
Hey guys

I have a few things which makes me think, when it comes to logical thoughts on lenses etc.

If i have a 50mm 1.4 lens on FF, will it give me the same depth of field like a 100mm 2.0 lens on FF?

My thesis: as focal length is a factor for the DOF, it would be logical when you double the amount of light coming through the lens, you would need half the focal length required to get the same DOF and bokeh (despite that every lens renders it different)

Is that true?

Then, why does fullframe sensors offer a better isolation with the same lens on crop? Cause, lets say you have a 50mm 1.4 lens: On FF thats effective 50mm FOV with great isolation at 1.4. If you put it on a crop sensor, the FOV becomes something around 80mm equivalent on FF. So the DOF and isolation is more as the same lens at same aperture. So if crop sensors has less DOF than same lenses on FF, it should be almost the same. But why isnt it like that?

Full frame will give you better isolation from the background with the same focal length because you move physically closer to the subject to frame the same shot.

Mavgirl wrote in post #16069926
Full frame will give you better isolation from the background with the same focal length because you move physically closer to the subject to frame the same shot.

Moving closer to the subject changes the perspective in an image. Comparing two different format cameras with the "same" image requires using the same camera location and different focal lengths (to keep the framing of the image the same on the two formats).

Nobody comparing two different point-n-shoot cameras, for example, would "foot-zoom". They would simply adjust the focal length for the same framing on the two cameras.

davidfarina wrote in post #16069477
Hey guys

I have a few things which makes me think, when it comes to logical thoughts on lenses etc.

If i have a 50mm 1.4 lens on FF, will it give me the same depth of field like a 100mm 2.0 lens on FF?

My thesis: as focal length is a factor for the DOF, it would be logical when you double the amount of light coming through the lens, you would need half the focal length required to get the same DOF and bokeh (despite that every lens renders it different)

Is that true?

Then, why does fullframe sensors offer a better isolation with the same lens on crop? Cause, lets say you have a 50mm 1.4 lens: On FF thats effective 50mm FOV with great isolation at 1.4. If you put it on a crop sensor, the FOV becomes something around 80mm equivalent on FF. So the DOF and isolation is more as the same lens at same aperture. So if crop sensors has less DOF than same lenses on FF, it should be almost the same. But why isnt it like that?

to frame the same area you'd have to get double the distance away in the 100mm shot

so 50mm f1.4 at 10ft=.65ft DOF
100mm at f2 at 20ft=.91ft DOF

the reason that FF offers better isolation is due to the fact that if you're looking to frame the same area in a shot, you either need to get physically closer with the same lens, or increase your focal length and shoot from the same spot

both of these will result in shallower DOF

also just a note, but the 100mm would be better at blurring far off distant backgrounds

Mavgirl wrote in post #16069926
Full frame will give you better isolation from the background with the same focal length because you move physically closer to the subject to frame the same shot.

Full frame will give you better isolation from the background with the same focal length assuming you move physically closer to the subject to frame the same shot. (note, this is not always possible in some situations (wildlife, landscapes).

Oh crap! Here we go again. If you really want to KNOW how DOF appears with different settings, follow these simple rules:

1.) Observe photograph.
2.) Change settings.
3.) Repeat.

Nothing to see here, folks. Please keep moving.

I have the 50 F1.2 and 100 F2, and quite frankly, the 100 will give a more pleasing images if framed the same.

Not only the background blur very good, but it's sharp from F2, where the 50 is a bit soft at 1.2. On a related note, perspective distortion is very different between the two lenses, so kind of hard to compare 1:1 images.

Every thread in every forum on this subject ends in about 5 different theories! Popcorn please!

dbeugel wrote in post #16070301
Every thread in every forum on this subject ends in about 5 different theories! Popcorn please!

Don't forget the salt!

A lot of people mean background and foreground blur amounts when they say "same amount of DOF". Keep in mind that they are different things.

Using a 200mm F4 lens at 20 feet and a 50mm f4 lens at 5 feet will get roughly the same DOF, but the 200mm shot will have 4x the background/foreground blur.

Tommy1957 wrote in post #16070198
Oh crap! Here we go again. If you really want to KNOW how DOF appears with different settings, follow these simple rules:

1.) Observe photograph.
2.) Change settings.
3.) Repeat.

Nothing to see here, folks. Please keep moving.

Hard to do when you haven't bought the lens yet or don't have both a FF and crop camera. Talking in relative terms is pretty useful.

tkbslc wrote in post #16070547
Hard to do when you haven't bought the lens yet or don't have both a FF and crop camera. Talking in relative terms is pretty useful.

Go through any DOF-related thread and find a consensus on ANYTHING. You will find about five different opinions, and six variations of each. Post an example that SHOWS one of the differences and people will argue about what you did wrong, WITH THE EVIDENCE RIGHT IN THEIR FACES!

There is only one way to learn anything about how aperture, distance, focal length and format affect DOF. That is to go out and shoot. Take a real photograph with two different focal lengths or two different apertures, then compare them. As I said:

Look at a photo.
Change settings.
Repeat.

That will get you somewhere. Debating the meaning of "blur" is a waste of time.

Few things, id really try tommys version out, but dont own a 50 1.4 nor a 100mm 2.0.

And most of you understood me wrong. Im not thinking that i am changing my distance to a subject. The framing wont be the same with a 100mm and a 50mm. I mean, when i use a 50mm at 1.4 20feet away to my subject, and then use a 100mm 2.0 again from 20feets away from my subject, will the depth of field be the same then? The framing will be totally different but theoretical spoken, shouldnt the DOF be the same then?

hope u understood me right now..

davidfarina wrote in post #16070638
Few things, id really try tommys version out, but dont own a 50 1.4 nor a 100mm 2.0.

And most of you understood me wrong. Im not thinking that i am changing my distance to a subject. The framing wont be the same with a 100mm and a 50mm. I mean, when i use a 50mm at 1.4 20feet away to my subject, and then use a 100mm 2.0 again from 20feets away from my subject, will the depth of field be the same then? The framing will be totally different but theoretical spoken, shouldnt the DOF be the same then?

hope u understood me right now..

DOF might be the same at that point, but the 100 F2 will give better background blur. You really should frame things the same.

I can give samples, since I have both the 50 f1.4 and 100 f2, but dont have any volunteer subjects right now.