Equivalent focal lengths

"Michelle Steiner" wrote in message
...
In article ,
"David J Taylor" wrote:

Because when you crop the captured image, you're increasing the
pixel size, thus reducing the sharpness.

If a CCD sensor is smaller than a full 35mm frome (36 x 24mm), then
the area which the sensor sees is just the same as cropping the 35mm
negative in an enlarger, or cropping a full-frame CCD sensor image in
software.

I'm not comparing the same focal length on different sensor or film
sizes; I'm comparing different focal lengths on the same film or sensor
size.

Exactly. Different film and sensor sizes are just that, different. It's
senseless to call it a crop or a magnification.

Greg

G.T. wrote:


Exactly. Different film and sensor sizes are just that, different. It's
senseless to call it a crop or a magnification.

One way or another, for comparative reasons or anything else, it is useful to
know the 'crop' factor or 'magnification' value between the standard 35mm format
and whatever smaller format digital sensor is used.

Most exp. for those who use the same lens on both film and DSLR's.

Cheers,
Alan.


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Michelle Steiner wrote:
In article ,
Alan Browne wrote:


This sub thread is in a semantic tangle over the word "crop". Don't
worry about it.


Do you mean that I should be anti-semantic?

As long as you're not anti-semitic.


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"Alan Browne" wrote in message
...
G.T. wrote:


Exactly. Different film and sensor sizes are just that, different.
It's
senseless to call it a crop or a magnification.

One way or another, for comparative reasons or anything else, it is useful
to
know the 'crop' factor or 'magnification' value between the standard 35mm
format
and whatever smaller format digital sensor is used.


Yes, knowing the conversion factor helps film SLR users pick lenses for
their digital SLRs.

Most exp. for those who use the same lens on both film and DSLR's.


My film SLR experience is limited so the conversion factor is meaningless to
me. To me there is no crop or magnification. I take a picture with my
Digital Rebel and it is what it is.

Greg

Dr. Joel M. Hoffman wrote:

My question is, if I put my current 50mm on a D7, what would its
equivalent focal length to a 35mm camera be?


It'd be equal to a 75mm lens.

Thanks.

Does that 1.5 : 1 ratio hold true for all focal lengths? Would my 85 mm
lens be equivalent to a 127.5 mm lens?


Yes, it's always x1.5, but you should also know that a 50mm lens on
your digital Nikon will not behave exactly the same way a 75mm does on
a film camera. The way to think of it is this: you're still shooting
with a 50mm lens, but you're always cropping to end up just using the
middle part of the picture.

In almost all cases, using a cropped sensor DSLR will use the best part of the
lens where there is the best resolution and the least linear distortion as well
as clearly avoidng vignetting from additional filters such as circ-pols. Given
the overall 'inconvenience' of using 35mm format lenses on cropped DSLR bodies,
these are considerable countervailing benefits.

In most cases, the 'cropped' performance of a specific lens will be better
performance than the 'equivalent FL' lens would be at comparable aperture and price.

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.

My 300 f/2.8 becomes a 450 f/2.8 or a 630 f/4 with the 1.4TC! Talk about going
up market real fast (with the TC, the res won't be as good as the 600 f/4 of
course, but damned good IAC). With the 2TC, it's a 900 f/5.6!!

My 80-200 goes to 120 - 300 f/8. Great for sports/nature.

In all cases above, of course, the sweetest part of the lens is used.

A minor negative is, that for the narrower FOV of the lens on a cropped sensor,
an excess of glass is presented which increases susceptibility to flare. This
is manageable of course by careful photographers.

Cheers,
Alan.


--
-- 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.

Michelle Steiner writes:

In article ,
"David J Taylor" wrote:

Because when you crop the captured image, you're increasing the
pixel size, thus reducing the sharpness.

If a CCD sensor is smaller than a full 35mm frome (36 x 24mm), then
the area which the sensor sees is just the same as cropping the 35mm
negative in an enlarger, or cropping a full-frame CCD sensor image in
software.

I'm not comparing the same focal length on different sensor or film
sizes; I'm comparing different focal lengths on the same film or sensor
size.

Actually, yes you are. The post to which you replied (wrongly) was
comparing 50 and 75mm lenses between film and cropped DSLR bodies. You
appear to be confused enough to start going on about 'optical
cropping'.

B

Michelle Steiner wrote:
In article ,
"David J Taylor" wrote:

Because when you crop the captured image, you're increasing the
pixel size, thus reducing the sharpness.

If a CCD sensor is smaller than a full 35mm frome (36 x 24mm), then
the area which the sensor sees is just the same as cropping the 35mm
negative in an enlarger, or cropping a full-frame CCD sensor image in
software.

I'm not comparing the same focal length on different sensor or film
sizes; I'm comparing different focal lengths on the same film or
sensor size.

OK, I wasn't. To me, crop refers to a cutting of the image, not a change
of focal length.

David

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.

Cheers,
David

"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 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?

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.

Doug

Ryadia wrote:
[]
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 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?

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.

Doug

Interesting, Doug. So for portraits....

- you want the same perspective as a 90mm lens (say), so that defines your
viewpoint.

- you now want the same FOV, so that defines the new focal length required
as 1.5 x 90mm, i.e. 60mm.

- you want the same DOF as your 90mm f/2.0, so you need an aperture of
what? f/1.3? (I'm unsure about this).

- so you need a 60mm f/1.3 lens on digital (crop) to get the same results
as you 90mm f/2.0 on film?

Back of the envelope agrees with what you say....

What are the implications of going the other way? Is 6 x 6cm an "easier"
format for portraiture?

Cheers,
David