21mm or 25mm? the Cropping factor from 18~28mm

[Einst_Stein]

Advertisement (gone after registration)

To decide the coverage of various wide angle lenses, I checked out Zeiss ZM lens specs.

I normalized the framing coverage to see the equivalent cropping factors. The following table is the summary.

 

I want to see how much do I loss in framing coverage if I choose ZM 25mm instead of ZM 21mm. this table shows about 1mm each side in the 24mm dimension or 1,5mm each side i the 36mm dimension.

 

I convinced myself that the difference between 21mm and 25mm is actually very small. However, the 28mm frame line is quite smaller than 25mm, it's about 2~3mm shortage each side in 28mm's frame line, or about 1~1.5mm short in M9's VF.

 

Based on this, besides the other subjectve factors, I'm decing to choose 25mm over 21mm for M9.

 

================================================================

Zeiss ZM Lens 18/4 21/2.8 25/2.8 28/2.8

================================================================

min. focus distance (M) 0.5 0.5 0.5 0.5

image ratio at min distance 23 21 18 16

Horizontal coverage at min distance (cm) 56 47 43 37

vertial coverage at min distance (cm) 84 71 65 55

equivalent frame size normalize to 18/f4 24x36 20x30 18x28 16x24

equivalent frame size normalize to 21/f2.8 29x43 24x36 22x33 19x28

equivalent frame size normalize to 25/f2.8 31x46 26x39 24x36 20x30

equivalent frame size normalize to 28/f2.8 36x54 31x46 28x42 24x36

=================================================================

[Advertisement]

The Leica Q3 is available for pre-order at B&H Photo Video and Adorama

[arthury]

I am not sure if you have seen or used the 21mm/2.8 Biogon ZM. It's been tested to death by many reviewers. Erwin Puts' review for the 21mm Biogon ranked it as like Leica or very close to it. I own this lens when it was first released a few years ago and I enjoyed every bit of it. I started liking wide Biogon since the Hasselblad SWC days and enjoyed its perfection in a smaller package in the ZM version.

 

It all depends on your application whether you need super wide lenses or not. If you do, 21mm is money well spent than getting a 28mm or a 25mm.

 

M9; 21mm/2.8 ZM Biogon, stitched, original image was around 108Mpix.

[Einst_Stein]

No, I'm not questioning Zeiss 21mm at all. I know it's quality.

I'm making decision between ZM 21 or ZM 25. I'm comparing the frame coverage.

 

According to Zeiss's data sheet, 21mm is about 5% wider than 25mm. This translates to 1.5mm wider on the wide side. To me then the 25mm is a better choice.

 

Your milage can be different.

[lars_bergquist]

The pertinent measure is not millimeters but degrees of angle. The acreage lost or found depends on the subject distance; very little wide angle photography is done at closest focus. And only you can decide what you need or want.

 

The old man from he Kodachrome Age

[jaapv]

I am not sure if you have seen or used the 21mm/2.8 Biogon ZM. It's been tested to death by many reviewers. Erwin Puts' review for the 21mm Biogon ranked it as like Leica or very close to it. I own this lens when it was first released a few years ago and I enjoyed every bit of it. I started liking wide Biogon since the Hasselblad SWC days and enjoyed its perfection in a smaller package in the ZM version.

 

It all depends on your application whether you need super wide lenses or not. If you do, 21mm is money well spent than getting a 28mm or a 25mm.

 

 

Well, yes, but by that reasoning an 18 would be even better, and then a 15, and a 12, etc...

[01af]

To decide the coverage of various wide angle lenses, I checked out Zeiss ZM lens specs. I normalized the framing coverage to see the equivalent cropping factors.

That's a bad method of making a decision about purchasing a wide-angle lens.

 

 

I want to see how much do I lose in framing coverage if I choose 25 mm ZM instead of 21 mm ZM. This table shows about 1 mm each side in the 24 mm dimension or 1.5 mm each side in the 36 mm dimension.

 

I convinced myself that the difference between 21 mm and 25 mm is actually very small.

The values in your table hold for the minimum focus distance. At or near infinity, you'll lose more. To arrive at the 25 mm's field of view on 35-mm format with a 21 mm lens, you'd have to crop the latter down to 20.16 × 30.24 mm. This is quite substantial. The difference between 21 mm and 25 mm lenses isn't huge but neither is it 'actually very small.'

 

So don't buy a 25 mm lens when 21 mm is what you actually want. And vice versa. In any case, both ZM lenses are very good, so in terms of image quality you can't go wrong with either.

[Einst_Stein]

Advertisement (gone after registration)

The pertinent measure is not millimeters but degrees of angle. The acreage lost or found depends on the subject distance; very little wide angle photography is done at closest focus. And only you can decide what you need or want.

 

The old man from he Kodachrome Age

 

Don't argue with me, talk to Zeiss. That's their post.

[Einst_Stein]

That's a bad method of making a decision about purchasing a wide-angle lens.

 

 

 

The values in your table hold for the minimum focus distance. At or near infinity, you'll lose more. To arrive at the 25 mm's field of view on 35-mm format with a 21 mm lens, you'd have to crop the latter down to 20.16 × 30.24 mm. This is quite substantial. The difference between 21 mm and 25 mm lenses isn't huge but neither is it 'actually very small.'

 

So don't buy a 25 mm lens when 21 mm is what you actually want. And vice versa. In any case, both ZM lenses are very good, so in terms of image quality you can't go wrong with either.

 

A data is a data, you choose how to interprete, and what makes the most sense to you.

Angle or mm, bear or infinite, fine. It's all based on the same facts.

 

I don't claim a certain interpretation is better than the other. This just happens to fit me. If you don't like, ignore it. If you find it's useful, enjoy.

[NZDavid]

Angle of view varies significantly for each millimeter of focal length for wide angle lenses.

 

18mm = 100º

21mm = 92º

24mm = 84º

28mm = 76º

 

Hope this helps!

[01af]

A data is a data, you choose how to interprete, and what makes the most sense to you. Angle or mm, near or infinite, fine. It's all based on the same facts.

When you assess the difference, or similarity, of coverages between different focal lengths on the data which refers to the minimum focusing distances then your facts are incomplete and your interpretations misleaded.

[lars_bergquist]

The economists use to append to their equation the reservation ceteris paribus – everything else being equal. What they usually forget to mention is that everything else is nearly always different.

 

No matter if you quote coverage in centimeters or in degrees, there is always that 'else', subject distance. So and so many square centimeters at closest focus is fine, but the numbers are different for each distance until you reach infinity, where coverage is infinite. Similarly, image angle is always given at infinity, because this is where it is at its maximum. It diminishes as you focus closer, because the distance from the sensor to the exit pupil increases. This is why the finder frames of a RF camera are strictly valid only for one focusing distance. The Germans call the phenomenon Bildfeldschwund. If anybody knows the proper English term, then do tell me please.

 

The Bildfeldschwund however does less drastic things to the image angle than the object distance does to the absolute coverage in square centimeters. So when we compare focal lengths, we do usually quote image angle at infinity – comparing apples to apples. A lens with a diagonal capture angle of 100° at infinity can behave quite differently on a M9 (closest focus 70cm), a M3 (1 meter) or a SLR camera (40–45cm). Of course you know this. But please think it through.

 

Or our discussions descend to pig-shearing level: Much screaming and little wool.

 

The old man from the Kodachrome Age

[01af]

So and so many square centimeters at closest focus is fine, but the numbers are different for each distance until you reach infinity, where coverage is infinite. Similarly, image angle is always given at infinity, because this is where it is at its maximum. It diminishes as you focus closer ...

... and the take-away, in the context of this discussion, is—the difference in coverages at minimum focusing distance is smaller than the same difference at medium or long shooting distances. So the conclusion that the difference between a 21 mm lens and a 25 mm lens was 'actually very small' is a misapprehension.

 

Which of course says nothing about which lens was 'better.' Which lens to choose depends on the subjects to shoot and on the intents and purposes of the photographer. I'm just saying that looking at the fields-of-view at minimum focusing distance definitely is a bad way to make a decision on which lens to choose ... well, unless the intended purpose actually is using them at minimum focusing distance only.

[algrove]

I still think you should sell your M9 since it's too "clumsy".

[Einst_Stein]

Angle of view varies significantly for each millimeter of focal length for wide angle lenses.

 

18mm = 100º

21mm = 92º

24mm = 84º

28mm = 76º

 

Hope this helps!

 

This is over simplified. I've the correction.

 

1. This is not what Zeiss published 25mm, for example is something like (90,80,54) if I remember right. The angle was rounded to degree There is ~0.5 degree error.

2. The framing coverage is not proportion to covering angle. It's somewhat close to tan(cover_angle/2). independent of the distance.

3. It's not tan(cover_angle/2), somewhat less than that. It's due to the stretch/distortion. Usually the wider the lens, the more the distortion, and further less than tan(cover_angle/2). Although Zeiss does not have much distortion on the Biogon, but We Leica fans can tell any difference. Very difference is screaming in our kin eyes.

[MarkP]

This is over simplified. I've the correction.

 

1. This is not what Zeiss published 25mm, for example is something like (90,80,54) if I remember right. The angle was rounded to degree There is ~0.5 degree error.

2. The framing coverage is not proportion to covering angle. It's somewhat close to tan(cover_angle/2). independent of the distance.

3. It's not tan(cover_angle/2), somewhat less than that. It's due to the stretch/distortion. Usually the wider the lens, the more the distortion, and further less than tan(cover_angle/2). Although Zeiss does not have much distortion on the Biogon, but We Leica fans can tell any difference. Very difference is screaming in our kin eyes.

 

 

Welcome, dear visitor! As registered member you'd see an image here…

Simply register for free here – We are always happy to welcome new members!

 

Don't make your and our lives so difficult!

 

Just decide whether you want an lens with ~90° (21mm) or ~82% (25) coverage.

There is a big difference between the two.

See if you can test them to see for yourself.

The M9 IQ is good enough that you could always frame for 25mm with a 21mm lens and crop later - two lenses in one.

 

[StephenPatterson]

Just strolling along, 'eh Mark? Why bother???

[MarkP]

Just strolling along, 'eh Mark? Why bother???

 

either trolling or the OPs name does not reflect capacity:D

[NZDavid]

This is over simplified...

 

Excellent, thank you. I love over simplification! We need more of it!

[Advertisement]

Hallo,

Look here: Interesting Leica gear

[jaapv]

Still, the actual coverage of the lens is only of interest when one uses the lens to “get more “ on the image. This is a very limited use of wide-angle lenses, as much the same can be obtained by cropping or stepping backwards. They only get interesting when they are used to get points of view that create interesting perspectives. And then the differences between the focal lengths really start to show.

[01af]

25 mm, for example is something like (90,80,54) if I remember right.

You don't.

 

 

The framing coverage is not proportion to covering angle. It's somewhat close to tan(cover_angle/2) ...

That's right. So the differences in 'framing coverages' are even greater than the differences of the angles seem to suggest.

 

 

... independent of the distance.

It does depend on distance ... and in a complex way. If you don't know the minutitae of the respective lens designs—in particular focusing methods and principal plane distances, among others—then you simply cannot make reliable conclusions about far-range coverage from data about close-range coverage. That's why close-range coverage is stated explicily in the tech specs in the first place ... you cannot compute them yourself accurately just from focal length and minimum focusing distance because they depend on more factors.