[quote name='Brightcolours' timestamp='1305659207' post='8451']
...the 85mm f1.8's are very affordable little gems in their line ups, giving a reasonable 135mm f2.8 full frame equivalent.
[/quote]
Ok, confused again..! <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/huh.gif' class='bbc_emoticon' alt='  ' />
Would you explain that, please, BC..? Someone..?
I knew about the FL multiples on APS-C (1.5x for Nikon DX)
I hadn't realised the aperture was subject to a multiple (or rather, division) as well... I think this is probably going to make my brain hurt <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='  ' />
Ian
[quote name='IanCD' timestamp='1305755685' post='8554']
I hadn't realised the aperture was subject to a multiple (or rather, division) as well...
[/quote]
Not the aperture itself, of course. It stays the same physically and in terms of metering. However, the depth of field is affected. A 85/1.8 lens on DX gives roughly the same depth of field (in the final image) as a 135/2.8 lens on FX.
-- Markus
Editor
opticallimits.com
[quote name='mst' timestamp='1305756934' post='8559']
Not the aperture itself, of course. It stays the same physically and in terms of metering. However, the depth of field is affected. A 85/1.8 lens on DX gives roughly the same depth of field (in the final image) as a 135/2.8 lens on FX.
-- Markus
[/quote]
Thanks Markus,
So that's about 2 stops (does it stay more or less constant?)
Which would have a detrimental impact on (shallow) DoF except that DoF gets shallower with increased focal length... so do the effects cancel each other out..?
It's actually more like 1 stop. So, a 85/2 would actually be the "real" equivalent to a 135/2.8 FX lens. Or one could argue that a 85/1.8 behaves more like a 135/2.5.
However, the difference between f/2.8 and f/2.5 (or f/1.8 and f/2 for that matter) is marginal in the real world. So, 1 stop difference as rule of thumb works quite well.
-- Markus
Editor
opticallimits.com
Lets take a 135mm f2.8 lens on full frame as starting point.
Focal length: 135mm
f-value: f2.8
Aperture: 135 / 2.8 = 48mm
Now lets figure out an equivalent lens on APS-C (using 1.5x crop factor in this calculation example).
To get the same field of view:
135 / 1.5 = 90mm
To get an equivalent DOF, the hole (aperture) has to be the same size.
So....
Focal length: 90mm
Aperture: 48mm
f-value: 90 / 48 = f1.86666667 (used the not rounded of aperture size in the actual calculation).
That is the "official" way to calculate it.
Now the fast way (and happens to give exactly the same results):
Just use the crop factor for both the f-value and the focal length to get the equivalents.
135 / 1.5 = 90mm
2.8 / 1.5 = 1.86666667
Simple, isn't it?
Also works the other way around:
APS-C 1.5x crop 85mm f1.8 lens
85mm x 1.5 = 127.5mm
f1.8 x 1.5 = f2.7
Equivalent on FF: 128mm f2.7 lens
APS-C 1.6x crop 85mm f1.8 lens
85mm x 1.6 = 136mm
f1.8 x 1.6 = f2.88
Equivalent on FF: ~135mm f2.8
The difference between APS-C and FF is about 1 1/3rd stop.
The difference between 4/3rds and FF is about 2 stops.
The difference between 4/3rds and APS-C is about 2/3rds of a stop.
[quote name='mst' timestamp='1305759017' post='8561']
It's actually more like 1 stop. So, a 85/2 would actually be the "real" equivalent to a 135/2.8 FX lens. Or one could argue that a 85/1.8 behaves more like a 135/2.5.
However, the difference between f/2.8 and f/2.5 (or f/1.8 and f/2 for that matter) is marginal in the real world. So, 1 stop difference as rule of thumb works quite well.
-- Markus
[/quote]
Not really, the difference is a bit more than 1... more like 1 1/3rd stop.
f1.8 and f2.8 actually really are the equivalents (more or less).
[quote name='IanCD' timestamp='1305757190' post='8560']
Thanks Markus,
So that's about 2 stops (does it stay more or less constant?)
Which would have a detrimental impact on (shallow) DoF except that DoF gets shallower with increased focal length... so do the effects cancel each other out..?
[/quote]
Mmm, not really - it's easier to think of it as of narrower field of view instead. You have 1.5x narrower FOV - it means you have to step back to get the same framing - DOF increases correspondingly. Being a bit of nit-picker I have to point out that it's slightly more than one stop (~1.08 stop for Nikon and ~1.14 for Canon) but the difference is rather marginal, so let's say it's one stop.
Actually, 85mm f/1.8 performs exactly as 135mm f/2.8 on Canon <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/cool.gif' class='bbc_emoticon' alt='B)' />
Edit: I see that Brightcolours already calculated that <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/laugh.gif' class='bbc_emoticon' alt=':lol:' />
[quote name='Lomskij' timestamp='1305761174' post='8567']
Mmm, not really - it's easier to think of it as of narrower field of view instead. You have 1.5x narrower FOV - it means you have to step back to get the same framing - DOF increases correspondingly. Being a bit of nit-picker I have to point out that it's slightly more than one stop (~1.08 stop for Nikon and ~1.14 for Canon) but the difference is rather marginal, so let's say it's one stop.
Actually, 85mm f/1.8 performs exactly as 135mm f/2.8 on Canon <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/cool.gif' class='bbc_emoticon' alt='B)' />
Edit: I see that Brightcolours already calculated that <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/laugh.gif' class='bbc_emoticon' alt=':lol:' />
[/quote]
So that makes it closer to 1 1/3rd stop... not 1.08 or 1.14 <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/wink.gif' class='bbc_emoticon' alt='  ' />.
A tad over 1 1/3rd stop for Canon APS-C, a tad under 1 1/3rd stop for "Sony" (Pentax, Sony, most Nikon APS-C).
[quote name='Brightcolours' timestamp='1305762564' post='8569']
So that makes it closer to 1 1/3rd stop... not 1.08 or 1.14 <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/wink.gif' class='bbc_emoticon' alt=' ' />.
A tad over 1 1/3rd stop for Canon APS-C, a tad under 1 1/3rd stop for "Sony" (Pentax, Sony, most Nikon APS-C).
[/quote]
Oops, forgot the square dependence. However after recalculation I still get that Canon is ~1.3 stop and Nikon is ~1.15 stop. Mind sharing how do you get that 1 1/3 stop figure?
[quote name='Lomskij' timestamp='1305768011' post='8572']
Oops, forgot the square dependence. However after recalculation I still get that Canon is ~1.3 stop and Nikon is ~1.15 stop. Mind sharing how do you get that 1 1/3 stop figure?
[/quote]
f2.8 to f2 is one stop... f2 to f 1.8 is 1/3rd stop.
For Canon, its f1.8 -> f2.88, a bit over 1 1/3rd.
For Nikon its f1.8 -> f2.7, a bit under 1 1/3rd. Actually... f2.726 for for instance a D300 (1.52x crop), f2.808 for a D3100.
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