Equivalent focal lengths

David J Taylor wrote:
[]
- you now want the same FOV, so that defines the new focal length
required as 1.5 x 90mm, i.e. 60mm.

Corrected 90mm / 1.5 = 60mm

David

"Ryadia" writes:

"David J Taylor" wrote in message
...

Not really - you have just the same picture as if you just used the
central part of a full-frame sensor with the 50 f/1.7 lens. By magnifying
and only using the central part you will magnify the any defects as well
(as a fraction of the total image), and make them more visible. Of
course, exactly /how/ visible they are will depend on the lens and sensor.

Cheers,
David

The crop factor is pretty confusing stuff when you are used to a portrait
lens being 90/100mm. The crop factor of a 50mm lens makes it "look" the same
size as an 80mm would but with the perspective of a 50mm lens!

No. Perspective is a function of the distance between lens's front
nodal point and the subject. And that's all.

If you crop the image from a 50mm lens to the point where you're getting the
same coverage as 75mm lens, and you are standing in the same place as you
would with a 75mm lens you will get *the identical perspective*.

It is this
perspective thing which gives traditional photographers the biggest amount
of drama. There is not the same opportunity for focus depth with a DSLR
using a 50mm lens as there is with a 35mm camera using a 90 or 100 mm lens.
When was the last time you saw a 65mm portrait lens.

Since 50mm lenses tend to be far faster than most portrait lenses, I'd
say that exactly the same opportunities arise. I argee that people
seem to be confused about perspective. You for one.

I used to pull focus on an eye with f2.0. The cheek or nose would soften and
conceal skin blemishes. Even at f1.4, a 50mm lens has too much DOF to pull
this off. I find now that to obtain the same results as I used to get, I
have to frame a shot using the same lens as I did with film but with greater
distance between me and the subject.

No, I rather suspect not. the f/1.4 to f/2.0 different should be just edging
on exactly the difference you'd need for your 1.5 crop factor to cancel out
and get exactly the same result.

B

David J Taylor wrote:

Alan Browne wrote:
[]

A case in point. My 50 f/1.7 will become a 75 (ish) f/1.7. So for
US$80 I get a lens with useability and performance that is close to an
85 f/1.4
which costs 8x as much.

[]

Cheers,
Alan.


Not really - you have just the same picture as if you just used the
central part of a full-frame sensor with the 50 f/1.7 lens. By magnifying
and only using the central part you will magnify the any defects as well
(as a fraction of the total image), and make them more visible. Of
course, exactly /how/ visible they are will depend on the lens and sensor.

The center of the lens is the least distorted, highest MTF part of the lens, as
any MTF graph will clearly show:
eg: http://www.photodo.com/pix/lens/mtf/MIAF5014.gif
(numbers on the bottom of the graph are radius (mm)from center at the sensor plane)

Cheers,
Alan.

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Ryadia wrote:

"David J Taylor" wrote in message
...

Not really - you have just the same picture as if you just used the
central part of a full-frame sensor with the 50 f/1.7 lens. By magnifying
and only using the central part you will magnify the any defects as well
(as a fraction of the total image), and make them more visible. Of
course, exactly /how/ visible they are will depend on the lens and sensor.

Cheers,
David


The crop factor is pretty confusing stuff when you are used to a portrait
lens being 90/100mm. The crop factor of a 50mm lens makes it "look" the same
size as an 80mm would but with the perspective of a 50mm lens! It is this

Perspective is independant of focal length. The perspective to a subject with a
600mm lens at a given point is the same as a 20mm lens at that point. The
difference is field of view.

perspective thing which gives traditional photographers the biggest amount
of drama. There is not the same opportunity for focus depth with a DSLR
using a 50mm lens as there is with a 35mm camera using a 90 or 100 mm lens.
When was the last time you saw a 65mm portrait lens?

The field of view of a 50mm lens on a 1.5 cropped sensor is identical to the FOV
on a 75mm lens (if such a lens existed.)

Cheers,
Alan


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-- [SI] gallery & rulz: http://www.pbase.com/shootin
-- e-meil: there's no such thing as a FreeLunch.

Alan Browne wrote:
[]
Not really - you have just the same picture as if you just used the
central part of a full-frame sensor with the 50 f/1.7 lens. By
magnifying and only using the central part you will magnify the any
defects as well (as a fraction of the total image), and make them
more visible. Of course, exactly /how/ visible they are will depend
on the lens and sensor.

The center of the lens is the least distorted, highest MTF part of
the lens, as any MTF graph will clearly show:
eg: http://www.photodo.com/pix/lens/mtf/MIAF5014.gif
(numbers on the bottom of the graph are radius (mm)from center at the
sensor plane)
Cheers,
Alan.

Agreed, but any defects will still be enlarged as a percentage of the
picture size if the image is cropped.

Cheers,
David

Gisle Hannemyr wrote:
[]
I would say that a 60 mm f/1.2 (~ 90 mm f/1.8) would be ideal, - but
that lens doesn't exist. Of those that exist, the Nikkor 85 mm f/1.4
(~ 128 mm f/2.1 is probably one of the better for control over DOF).

So no modern-day equivalent to the 58mm f/1.2 then?

http://www.mir.com.my/rb/photography/hardwares/speciallenses/55mmnotcnikkor.htm

Cheers,
David

Gisle Hannemyr writes:

Bruce Murphy writes:
No, I rather suspect not. the f/1.4 to f/2.0 different should be
just edging on exactly the difference you'd need for your 1.5 crop
factor to cancel out and get exactly the same result.

Ryadia's 20D has a 1.6 factor, not 1.5. With a 1.6 crop you need to
open up 1 1/3 stop to get the same results.

I keep forgetting about people will silly cameras.

If we do the math for a 1.5 factor (Nikon, Pentax, Fuji, et. al),
we need to open up 1 1/5 stop to get the same results at the same
FOV as film. So a 50mm f/1.4 on a D70 becomes the equivalent of
a 75 mm f/2.1 - and we are still too wide and too slow to match
a 90/100 mm f/2.0.

I consider 75mm to be a fairly nice portrait length, actually.

So what is the "ideal" portrait lens for digital?

I would say that a 60 mm f/1.2 (~ 90 mm f/1.8) would be ideal, - but
that lens doesn't exist. Of those that exist, the Nikkor 85 mm f/1.4
(~ 128 mm f/2.1 is probably one of the better for control over DOF).

I was using a 55mm f/1.2 on my D70 two weeks ago.

B

David J Taylor wrote:


Agreed, but any defects will still be enlarged as a percentage of the
picture size if the image is cropped.

Which defects?

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Alan Browne wrote:
David J Taylor wrote:


Agreed, but any defects will still be enlarged as a percentage of the
picture size if the image is cropped.

Which defects?

Which ever ones the lens has.

David

Gisle Hannemyr wrote:

Bruce Murphy writes:

"Ryadia" writes:

"David J Taylor" wrote:


Not really - you have just the same picture as if you just used
the central part of a full-frame sensor with the 50 f/1.7 lens.
By magnifying and only using the central part you will magnify the
any defects as well (as a fraction of the total image), and make
them more visible. Of course, exactly /how/ visible they are will
depend on the lens and sensor.


The crop factor is pretty confusing stuff when you are used to a portrait
lens being 90/100mm. The crop factor of a 50mm lens makes it "look" the same
size as an 80mm would but with the perspective of a 50mm lens!


No, I rather suspect not. the f/1.4 to f/2.0 different should be
just edging on exactly the difference you'd need for your 1.5 crop
factor to cancel out and get exactly the same result.


Ryadia's 20D has a 1.6 factor, not 1.5. With a 1.6 crop you need to
open up 1 1/3 stop to get the same results. If we do the math, we find
that a 50mm f/1.4 on a 20D becomes the equivalent of a 80 mm f/2.2 on
a film camera. If what you want is 90/100 mm f/2.0, that is too wide
and too slow.

What math are you doing?

Recompute for the DOF "standard" paper size at print (for printed 8x10 at 5 l/mm
or at the extreme 8 l/mm). That is what DOF is all about: perceptible sharp
detail in the printed image. So, given the smaller image area, when it is blown
up to a reference size, the DOF will be shallower as the source image (camera
sensor) is smaller.

Cheers,
Alan


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-- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm
-- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm
-- [SI] gallery & rulz: http://www.pbase.com/shootin
-- e-meil: there's no such thing as a FreeLunch.