Tamron 28-75 vs. Nikon 24-85 AF-S: Depth of Field

I am deciding between the Tamron 28-75/2.8 versus the Nikon 24-

85/3.5-4.5. The Tamron appeals to me not because of improved low-

light capability but because the larger constant aperture allows for

shallower depth of field. Otherwise, I prefer the range of the

Nikkor. To determine the effect of the Tamron's larger aperture on

depth of field, I ran a test with my 50/1.4. I took a series of the

types of photos where I would typically select a large aperture

(close-ups of flowers, portraits with cluttered backgrounds, scenics

shooting down a fence, etc.). The first shot in each series was

taken at f2.8, the second at f4. The only variable was the aperture

(and shutter speed, but it was never slow enough to cause a

problem). When I compared the results (4x6 prints), I discovered

that the depth of field between f2.8 and f4 was almost

indistinguishable. As expected, it was most noticeable with the

close-ups, but even then the difference was slight. My question:

was my methodology flawed, or is there in fact little visible

difference in the depth of field between f2.8 and f4?

In close-up photography, there is precious little depth of field, regardless of the aperture. So it does not surprize me that you saw little difference in DOF between f/2.8 and f/4. Part of the challenge in macro photography is making the most of the DOF you have. That's where good technique comes in. In the specific case of the lenses you mention, you are going for minimum DOF (at your lenses' largest aperture) and therefore greatest background blur. To evaluate, look at the backgrounds to see how blurred-out they are at the different apertures.

 

The goal of course is the very blurry background with a tack sharp foreground. To get that, many macro shooters use a blurry background they prepare in advance by having a very blurry shot of background printed up and placing it behind the subject of the photo. That way, you can use smaller apertures on the lens -- to take full advantage of whatever DOF you can get -- and still get the blurry background provided by a lerger aperture and a more distant background.

Your observations sounds right.

* The closer your subject is (focusing distance), the more you will see difference in depth of field.

My opinion is that on the shorter focal lenghts, it is not much of a difference. But when you are at 75/85mm, you will definitly see a bigger change in depth of field between f2.8 and f4.5. It is also the question on how far away your background is and how it looks, but that is your own technique or subject, not the lens doing the difference.

I say: If you want a wide range of focal lengths, go with the 24-85. If you need the speed, go with the 28-75. If you want shallow depth of field, you always have your 50mm f1.4....

How close were you to the flower?

 

The DOF at f/4 is 41.4% deeper than at f/2.8. But at the minimum focus of the 50mm f/1.4, that's the difference between a centimeter and 1.44 cm.

Thank you, everyone, for your responses. As for using the 50/1.4 as my shallow DOF lens, there are two problems: (1) it is a Zeiss lens (I don't currently own any Nikon equipment), and (2) my goal is to carry a single lightweight lens/SLR combination, plus a small P&S, during my travels this summer. That is the dilemma: I want the range of the 24-85, but, with my Contax gear, I frequently shoot at large apertures, so I also want the largest aperture possible in a lightweight package. My experiment was designed to uncover how much "selective focus" ability I give up as between the Tamron and the Nikkor. 41% sounds like a lot, but in reality, it isn't that noticeable, at least not in my series of shots.