Question about long lenses

[Jeffrey A. Steinberg]

(35mm lens format used for questions).

Can someone let me know if this logic is correct?

I am looking at the Cannon 400mm f5.6. I have a 300mm f4 and a teleconverter. The questions below are for the lens without the TC.

The question is this: as the lens gets longer, does the aperature necessary to create a "fuzzy" background (i.e. shallow DOF) get smaller such that a 400mm f5.6 lens will have about as muted a background as a 300mm lens at f4?

My logic is that the hyperfocal (depth of full focus) distance becomes larger as aperture becomes smaller so by inference doesn't hyperfocal distance (and thus depth of field) become smaller as aperture increases do sort of equalize the decrease in maximum aperture size for the longer lens with its increase in mm's)?

I don't want to buy the 400mm lens if it doesn't have the same relative ability to drop the background into out-of-focus to make sure the subject is not distracted with background objects.

Thanks in advance and I hope this question makes sense.

 

[JBrunner]

(35mm lens format used for questions).

Can someone let me know if this logic is correct?

I am looking at the Cannon 400mm f5.6. I have a 300mm f4 and a teleconverter. The questions below are for the lens without the TC.

The question is this: as the lens gets longer, does the aperature necessary to create a "fuzzy" background (i.e. shallow DOF) get smaller such that a 400mm f5.6 lens will have about as muted a background as a 300mm lens at f4?

My logic is that the hyperfocal (depth of full focus) distance becomes larger as aperture becomes smaller so by inference doesn't hyperfocal distance (and thus depth of field) become smaller as aperture increases do sort of equalize the decrease in maximum aperture size for the longer lens with its increase in mm's)?

I don't want to buy the 400mm lens if it doesn't have the same relative ability to drop the background into out-of-focus to make sure the subject is not distracted with background objects.

Thanks in advance and I hope this question makes sense.

Makes perfect sense. The "fuzzies" are not just a function of aperture/focal length/image area.
Lens bokeh is also correlated to the distance of the background from the focused subject, and the distance of the focused subject to the focal plane. That said, you will have little trouble getting a blurry background with a 400mm f5.6 in most instances. Wide open on the 400 the effect on the DOF is only 1/2 stop when you take the extra 100mm into account.

 

[ath]

Except for Macro, the depth of field is (approximately) depending only on aperture and magnification. If you fill the frame with the same object using the 300mm and the 400mm both will have the same DOF at the same apertures.

 

[Christopher Walrath]

The 400 would have a maximum aperture diameter of 71.5mm at f/5.6. The 300 would be 75mm at f/4. Though the depth of filed might be somewhat shallower with the 400, you also lose depth differentiation. The DOF would prob be about the same.

 

[Jeffrey A. Steinberg]

Thank you all for confirming my thoughts. Now I just have to get it passed the internal CFO if you know what I mean.

 

[ath]

To be precise, Jeffrey, I didn't confirm your thoughts. I stated, that they are wrong.

Both 300mm and 400mm have the same DOF at 5.6, provided, that the magnification is the same (i.e. you have compensated the smaller focal length by moving further to the subject).

This can be calculated from the basic lens equations.

 

[DBP]

Using f/Calc, at a distance of 10 meters, I get a DoF of 17cm for a 400mm lens at f/5.6, a DoF of 21cm for a 300mm lens at f/4, and a DoF of 15cm for a 420mm lens at f/5.6 (equivalent to a 1.4 teleconverter on the 300/4).

 

[Jeffrey A. Steinberg]

Yes, sorry. I meant the conclusion seems to be about the same; my explanation was wrong.

Thanks, all.