Is my (eek) maths right here?

Ksanti

Ksanti

Ksanti

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WARNING: Lots of poorly organised maths thinking here :D

I'm a couple of months away from hitting my savings target for my next lens for my 5D; this one being probably my last for a while - a portraiture lens. I'm deciding between a Sigma 85 f/1.4 and Canon 135 f/2L. I realise the Sigma will walk all over the Canon in low light.

Been using DOFmaster to work out a couple of DOF things:
The Sigma would give shallower DOF for like-for-like images (i.e. more or less same composition).
Sigma:
85mm
f/1.4
8.5 feet subject distance
Total DoF: 0.25 feet

Canon:
135mm
f/2.0
13.5 feet subject distance
Total DoF: 0.35 feet

Perspective aside, is the assumption that focal length A/focal length B = subject distance A/subject distance B correct?

Now I do like bokeh, but this lens will most likely be used more for planned shooting than spontaneous shooting so it would be possible to set stuff up specifically if I want effect; namely the Brenizer "bokeh panorama" method. I've been toying with these for a while, and I'm wondering if the following maths is also right (for working out what DoF would be in a 135mm-shot image, using the Brenizer method to bring it back to an 85mm perspective):

Sigma:
85mm
f/1.4
8.5 feet subject distance
Total DoF: 0.25 feet

Canon:
135mm
f/2.0
8.5 feet subject
Total DoF: 0.14 feet

This would give the 135mm's 85mm equivalent images the same look as an 85mm f/0.x lens (DoFMaster only goes down to f/1.0 which give 0.18ft DoF).

To me the maths makes sense, as focusing at the same distance means the same subject distance which should in theory mean the same equivalent focal length once the panorama is stitched together. BrettMaxwell's calculator says that using a 135 f/2.0 lens at an equivalent of 85mm, gives an equivalent aperture of f/1.2xx, but I /think/ that might be different in that it doesn't adjust subject distance for each focal length i.e. you'd have a normal 135 frame with extra around the outside, and not move closer to get the panorama.
 
135/2=67.5
85/1.4=61

Thus the 135mm lens has a slightly larger aperture, thus more "bokeh" from the same position. So in a "bokeh panorama" it technically has the advantage. However the extra framing width on an 85mm would let you get closer neglecting this (slight) advantage in spontaneous shooting.

They will both provide enough bokeh. Therefore perhaps consider the other factors first - focal length, build quality, auto focus, image quality etc. etc. and whatever is most important to you and use that to choose as this is a slightly apples and oranges thing you have going on!!
 
Yeah I know it is, it's just nice to know that the 135 would have the DoF capabilities there in its back pocket if I did decide to use the panoramas; plus the 85 might well be a little short for those uses.

I know the 85 1.4 would give narrower DoF for spontaneous shooting but tbh I use my 50 1.4 for that.

I am aware that it's a bit apples and oranges, but it becomes worth comparing the two when you can only have the money for one apple or one orange :) I'm not basing my entire decision off of the extreme depth of field capabilities of one or the other, but I just wanted to check my maths for it.
 
The difference in bokeh between an 85/1.4 and 135/2 is pretty negligible so I'd almost forget that as a factor. They both have huge apertures - if you've got the 50mm, maybe the 135 compliments that a little bit better...?
 
Yeah I am leaning towards the 135 overall; better build, QC, AF, sharper. Less DoF for equivalent frames (if my maths for the first part was right), yes, but focuses closer and the extra length means I can do more extreme Brenizer panoramas when need be without getting into stupidly wide equivalent focal lengths like 14mm. :)
 
The Canon EF 135mm f/2.0 L was the last lens I bought. To put it plainly it is an incredible lens, the sharpest I own. Just do a search for reviews on it, you will not be disappointed.

Also thank you for enlightening me as to what the Brenizer method is, looks a very cool technique.
 
Longer focal lengths magnify oof areas, which gives the impression that the DOF is shallower but it actually isn't, at the same framing, a 135 F2 will have the same DOF as a 85 @ F2, but the bokeh on the 135 will look a little more creamier as it magnified.

135 is a bit tight for most scenarios imo, and is the type of lens that would only occasionally come in handy, for me at least.
 
OP - You are looking at this too much from a technical aspect.

Look up pictures each lens has taken then buy which ever one you fancy, both will be good.
 
Phate said:
OP - You are looking at this too much from a technical aspect.

Look up pictures each lens has taken then buy which ever one you fancy, both will be good.
Click to expand...

Totally agree, both lenses will produce great pictures with great bokeh the decision here is what focal length will be more usefull 85mm or 135mm and only you can really know the answer to that. Also to say the 85mm will totally kill the 135mm in low light is a massive exageration the difference bettween f2.0 and f1.4 isn't really all that big.
 
also remember that wider lenses dont require faster shutter speeds for bding handeld.
I will base this on the 85 1.8 i have and the 135 f/2 which i also have.

If you are shooting low light i can hand hold the 85mm at lower shutter speeds than i can with the 135mm. Why, weight, 1/focal length rule etc... rather use my reebok zoom get the frame right on the 85mm and take the shot. rather try shooting at 1/80 than 1/130 ish (cant remember what the closest to 135 is on the camera). however i much prefer the bokeh on the 135L. For the sigma 85mm the images i have seen look really impressive on the bokeh so i dont think you will be dissapointed anyway!
 
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