Camera Labs

Hi.
i wanted to know if the depth of field of a 100mm f2.8 lens gets more narrow with a x1.4 extender. im aware that x1.4 extender reduces the aperture by 1 stop but does this mean that only to the quantity of light passing through the lens is reduced or also the depth of field is changed.
since if one were to get an effective focal length that is longer than the original with the same distance from the subject the background should be more blurred right?

i found conflicting answers about this if you have experience with this or have a reliable source i would very much like to know.
thanks

I use this site to calculate this stuff:

http://www.tawbaware.com/maxlyons/calc.htm

For a given distance, DoF decreases with the square of the focal length and increases with the aperture value.
That means you should get a smaller DoF with the extender

The 1.4x extender gives you a 140mm f/4 (roughly)

Example
Distance: 2 m, focal length 100mm, apeture 2.8 -> DoF = 4 cm
Distance: 2 m, focal length 140mm, apeture 4 -> DoF = 2.9 cm

So yeah, you should get a shallower DoF

Anyone, please correct me if I'm wrong though!

_________________
Canon DSLRs: 5D Mark III, 7D
Canon lenses: 17-40/4L, 24-105/4L, 24/1.4L II, 50/1.4, 85/1.8, 100/2.8L Macro, 70-200/2.8L IS, 100-400L
Other stuff: Canon Extender 1.4x II, Canon 580 EX II, Canon 430EX II, Velbon Sherpa 600R
Apple: MacBook Pro 15", iMac 27", iPad III, iPhone 4S, Apple TV

My deviantART

Just a word of caution: The DoF is no indication of the amount of background blur. Here is an article I find useful to help understand the variables involved.

_________________
Canon DSLRs: 7D, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 150 macro, 120-300 f/2.8 OS, Celestron 1325/13
Tinies: Sony HX9V.

ic so the FOV for the extender with regards to DOF is still taken with 1 stop slower

@popo
but since we are comparing the same lens with and without an extender it comparison is logical, or at least as far as im aware of, dont you agree?

also very nice detailed and referenced article thanks for that

I believe janern answered the original question, that you should consider a 100mm f/2.8 lens with 1.4x extender to be a 140mm f/4 lens. The caluclator then gives you the DoF results at that setting.

But I saw your question in two parts, one regarding the DoF, and one regarding the background blur. Even if you keep the same DoF by changing aperture or subject distance for example, the apparent background blur can vary. That is what I wanted to highlight.

_________________
Canon DSLRs: 7D, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 150 macro, 120-300 f/2.8 OS, Celestron 1325/13
Tinies: Sony HX9V.

aaa ok now i get what you meant
thanks both

The article was definately an interesting read. Thanks for posting, popo

_________________
Canon DSLRs: 5D Mark III, 7D
Canon lenses: 17-40/4L, 24-105/4L, 24/1.4L II, 50/1.4, 85/1.8, 100/2.8L Macro, 70-200/2.8L IS, 100-400L
Other stuff: Canon Extender 1.4x II, Canon 580 EX II, Canon 430EX II, Velbon Sherpa 600R
Apple: MacBook Pro 15", iMac 27", iPad III, iPhone 4S, Apple TV

My deviantART

Yeah i agree with janern a good read popo

_________________

Canon 400D. BG-E3. canon 18-55mm. canon 28-80mm. sigma 70-300mm APO macro.