DOF difference between the 85mm 1.2 and the 50mm 1.2

There are two thing affecting DOF, aperture and magnification.

Nick_Reading.UK wrote in post #16739657
I calculated the 50mm at 5metres and the 85mm at 8 metres and the 85mm has less DOF.. Wow!!!

In this example the magnification is different. Compare 50mm at 5m and 85mm at 8.5m for a comparison. If you use 85mm at 8m then it's a larger magnification than 50mm at 5m.

In theory, if you take a photo of a 2D subject, at same magnification and same aperture, DOF is the same, but for a theoretical 2D subject there is no DOF. For a 3D subject, changing the distance and FL will keep magnification the asme only for a theoretical 2D area of the subject (i.e. the precise spot you are focused on), so different parts of a 3D subject have a different magnification as a result.

Nick_Reading.UK wrote in post #16739648
The 85mm seems to have less DOF.. I am wondering why. Maybe I calculated incorrectly

Go about 3/4 down the page for an explaination
http://www.the-digital-picture.com …SM-Macro-Lens-Review.aspx

clarnibass wrote in post #16740529
There are two thing affecting DOF, aperture and magnification.

I had to think about this a bit, but then realized magnification encompasses the other 2 attributes that affect DOF: focal length and distance.

The only other caveat to all of this is that 2 of the 3 factors affecting DOF won't affect perspective, but changing distance will. This means that if you use distance to control DOF, you won't end up with the same content in your photos that you would if you use focal length.

It can sometimes be difficult to compare different photos across different settings, if perspective is different.

In general, when you frame your subjects the same, with the same aperture, you'll end up with the same depth of field. Now this doesn't mean you'll end up with the same photograph, as the background compression will be different.

Here's the photo that windpig is referencing out of the macro lens reviews that shows exactly that.
http://media.the-digital-picture.com …ength-Background-Blur.jpg

So the real question here, is what are you trying to achieve? Are you looking for background separation or are you looking for a narrow DoF on your subject? Most people want the former, not the latter in the majority of shots they take.

TeamSpeed wrote in post #16739963
This is something I put together a long time ago, maybe it would be helpful here?

I registered JUST to tell you how amazingly helpful in visualizing this set of images are!

werds wrote in post #16740873
I registered JUST to tell you how amazingly helpful in visualizing this set of images are!

Thank you, I did it years back for somebody that needed help with DOF, and I simply could not explain it well enough in words, where others on the board are much better. Welcome to POTN!

vengence wrote in post #16740864
In general, when you frame your subjects the same, with the same aperture, you'll end up with the same depth of field.

Pretty much. It doesn't take too much hands-on time to realize that, for a given f-stop, no matter the FL, framing your subject in a particular manner will result in predictable DoF. A head shot at 1.2/1.4, for instance, isn't going to get the full face in focus - hell, it won't even get both eyes in focus unless you're shooting them dead-on, putting both eyes in the same plane.

werds wrote in post #16740873
I registered JUST to tell you how amazingly helpful in visualizing this set of images are!

Hay, welcome !!!!!!

TeamSpeed wrote in post #16739963
This is something I put together a long time ago, maybe it would be helpful here?

It's a great image. If you ever update that image, you might add red rectangles to show the relative framing. Or perhaps change the subject's size or something.

vengence wrote in post #16741090
It's a great image. If you ever update that image, you might add red rectangles to show the relative framing. Or perhaps change the subject's size or something.

Good idea, I wonder if I still have my PSD for this laying around...

I was just trying to show DOF relative to a focal point (using the people) with the diagram, but definitely the framing would be different for 2 of the 3 areas.

TeamSpeed wrote in post #16740789
I had to think about this a bit, but then realized magnification encompasses the other 2 attributes that affect DOF: focal length and distance.

Magnification has all attributes that affect DOF in it, except aperture.
The magnification is determined by FL, distance, sensor size and print/display size.
Those DOF calculators are based on some specific print size viewed from a specific distance, that I guess is a standard that I forgot.

This is a great calculator:
http://toothwalker.org​/optics/vwdof.html

It will calculate/compare DOF, as well as background blur at chosen bg distances.

Anyway, for the same framing (i.e. same number of respective focal lengths shooting distance), the DOF is the same for any two equal apertures.
Said another way, for the same framing the DOF depends only on aperture, irrespective of FL.

gasrocks wrote in post #16739663
Be honest with us and yourself, how often are you going to shoot either lens at 1.2?

Almost always? I don't see why anyone would buy either lens if you were not intending to use them wide open quite often.

draculr wrote in post #16742419
Almost always? I don't see why anyone would buy either lens if you were not intending to use them wide open quite often.

I'm mixed feelings on this. If the lens was 1.8 or 2.8 absofriggenlutely. 1.2 though, it's just too damn narrow DoF on most shots. having the eyes in focus and the eyelashess OOF doesn't make for good photography. That said, I don't buy a 1.2 lens to stop down to 2.8 or 4 most of the time either.

vengence wrote in post #16742554
I'm mixed feelings on this. If the lens was 1.8 or 2.8 absofriggenlutely. 1.2 though, it's just too damn narrow DoF on most shots. having the eyes in focus and the eyelashess OOF doesn't make for good photography. That said, I don't buy a 1.2 lens to stop down to 2.8 or 4 most of the time either.

I agree with draculr. Almost always. Neither are really good to get in a tight headshot (OOF eyelash scenario), and it would make no sense to shoot wide open in such a scenario, but environmental and candids, 1.2 all the way.