[stevebrot]

It seems logical to assume the focal length is pinned 1:1 to correspond to area of coverage of said lens focal length.

Area of coverage varies by distance and guide number is distance, literally, the distance at f/1.0 where the flash intensity will be sufficient for proper exposure within the scope of a 100% viewfinder field (the object frame). Yes, doubling the flash zoom will cover the square root of the area assuming the subject distance is the same as the old guide number with a 100% efficient flash zoom. Otherwise, the coverage area stays the same at the new guide number distance. I am not sure I have the calculation right, but doubling the focal length should work out to about +1 EV. In the real world coverage at the GN distance tends to decrease from the expected with increased zoom setting, however.

The Pentax P-TTL Flash Comparison site has a page dedicated to comparison diagrams showing composite diagrams for flash horizontal coverage angle out to the GN distance for all of several different flashes' supported zoom settings. Below is the diagram for the AF540FGZ (v1).

Addendum: The zoom settings overlaid in the diagram are (left to right) 24mm, 28mm, 35mm, 50mm, 70mm, and 85mm. Note that distance increase for 50mm over 24mm is somewhat less than expected (actual is 45m and 32m respectively).

When viewed together with those from other similar flashes, one can get an idea of how coverage is managed from one flash to the next. To see several at a glance...

Flash Burst Profiles - Pentax P-TTL Flash Comparison (Hint...the non-zoom flash serve as a reference point for the others.)

As noted on the two sites linked above, this is not something that lends itself to calculation or even estimation, but it best done by reliance on a flash meter, printed tables, GN multipliers, or the scales displayed on some flash model's rears.


Steve

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Last edited by stevebrot; 09-22-2020 at 11:15 PM. Reason: Corrected math...again

[swanlefitte]

Those flash profiles are interesting. It looks like the less powerful zooms perform better in proportion to the more powerful if the profiles are consistent. Are they?
Look at the pentax 540. Very strong not zoomed vs the metz 50 about half that. Now zoomed they seem much closer. Does the metz zoom more? I wish the 50, 105, 135, 200 was a different color to see how settings differ. I suspect the metz is at 200 farthest out and the pentax is at only 150.

[stevebrot]

Those flash profiles are interesting. It looks like the less powerful zooms perform better in proportion to the more powerful if the profiles are consistent. Are they?
Look at the pentax 540. Very strong not zoomed vs the metz 50 about half that. Now zoomed they seem much closer. Does the metz zoom more? I wish the 50, 105, 135, 200 was a different color to see how settings differ. I suspect the metz is at 200 farthest out and the pentax is at only 150.

The zoom ranges vary between flashes. PF member @mattdm is responsible for the diagrams. With any luck he may drop in. Opps...I guess it has been awhile since he has been active here.


Steve

[swanlefitte]

Pentax 540 is 24-85
Metz 24-105
Took a bit of googling to find.
As I suspected it is hard to compare because it is difficult to cross reference the same zoom.
I had expected the pentax 540 to zoom to 150 because the cheaper 360 goes to 150. Is this comparable to flagships not having a dedicated flash? If you go flagship flash you will go off camera instead of zoom at 200?

[Apet-Sure]

Also the front fresnel lens is molded plastic, which does not refocus with zoom.

In the Vivitar 283 system, you actually put different fresnel lenses into the holder. But you only get 4 choices - 24mm, 28mm, 70mm, and 135mm. I would sometimes use the 24mm lens as a diffuser to soften the flash effect since it was, ah, diffusey. (Translucent, that's the word.) I've used empty cake frosting containers on other flashes.

[stevebrot]

In the Vivitar 283 system, you actually put different fresnel lenses into the holder. But you only get 4 choices - 24mm, 28mm, 70mm, and 135mm. I would sometimes use the 24mm lens as a diffuser to soften the flash effect since it was, ah, diffusey. (Translucent, that's the word.) I've used empty cake frosting containers on other flashes.

Yep, that definitely works!

We tend to forget that the label really means 24mm+, 28mm+, 70mm+, and 135mm+. By inference the naked reflector is probably 35mm+, something I should test on one of my film cameras.* The Exakta VXIIa is loaded with expired Fuji 400 B&W and I do have a 35mm lens for that camera...

FWIW...the built in zoom on the contemporary Viv 285 only goes to GN 140(ft) at 105mm setting compared to the Viv 283 GN 156(ft) with its 70mm lens attached.


Steve

* I, quite sadly, do not have a FF digital...

[Class A]

Crop factor is not a consideration and should not be. The focal length aspect of the tables indicates the relative area of illumination only. The actual illuminance for a given distance should be the same regardless of crop.

Well, it matters whether a certain zoom setting of a flash is expressed with reference to an FF or an APS-C sensor.

A 50mm flash zoom head setting designed to work with an FF camera, should be increased to a 75mm setting, if an APS-C camera is used.

The Metz 58 AF-2 has a respective crop factor setting to account for that fact.

As along as all GN specifications assume the same FF-based focal length figure for a head's zoom setting, there is no issue, of course.

N.B., GN measurements by manufacturers are typically not comparable because some flashes deliberately introduce more "hot-spotting" at higher zoom settings in order to result in a higher flash meter reading at the centre of the illuminated field. In other words, one can artificially drive up the advertised GN value simply by designing a zoom head that upon zooming not only reduces the illuminated field but also increases the light intensity at the centre. GN specifications would be easier to understand and compare if the homogeneity of illumination remained perfect at all zoom settings, but in reality it doesn't.

This serves as a reminder that measuring the power of light sources should in almost all cases not be attempted by taking a single light meter reading.

[stevebrot]

Well, it matters whether a certain zoom setting of a flash is expressed with reference to an FF or an APS-C sensor.

A 50mm flash zoom head setting designed to work with an FF camera, should be increased to a 75mm setting, if an APS-C camera is used.

The word "should" may safely be replaced by the word "may". FWIW, my comment was referring to guide numbers, not the flash zoom settings.

N.B., GN measurements by manufacturers are typically not comparable because some flashes deliberately introduce more "hot-spotting" at higher zoom settings in order to result in a higher flash meter reading at the centre of the illuminated field. In other words, one can artificially drive up the advertised GN value simply by designing a zoom head that upon zooming not only reduces the illuminated field but also increases the light intensity at the centre. GN specifications would be easier to understand and compare if the homogeneity of illumination remained perfect at all zoom settings, but in reality it doesn't.

This serves as a reminder that measuring the power of light sources should in almost all cases not be attempted by taking a single light meter reading.

Thanks for the succinct statement of what this thread has been waltzing all around without addressing directly.


Steve

[Class A]

FWIW, my comment was referring to guide numbers, not the flash zoom settings.

Yes, but since published guide numbers depend on the (max.) zoom setting, it matters what a zoom setting of "200mm" really means.

[stevebrot]

Yes, but since published guide numbers depend on the (max.) zoom setting, it matters what a zoom setting of "200mm" really means.

Yep...It really means coverage adequate for FF angle of view (horizontal and vertical, 3:2) at 200mm and longer at a GN of ##. Similarly, the same published maximum guide number applies for APS-C, but for 135mm and longer at that flash zoom setting.

When using manual flash technique with formats larger than 35mm FF, one must always be aware of crop factor as well as aspect ratio when selecting flash zoom setting; either that or make use of a flash meter. Manuals for Pentax-brand flash provide a table of appropriate flash zoom conversions for APS-C, 645 film (4:3), 645 digital (4:3) and 6x7 film; also included are guide number tables for those formats at the various zoom settings.

The manuals for generic speedlights generally do no include guide numbers for other than a single zoom setting and then only for 35mm FF.

(I know you know this stuff...included for those who don't.)


Steve

[swanlefitte]

Apparently different flash models work differently when auto zoom is used. A nikon sb600 always zooms to the lens size but the sb700 would zoom to 80mm with an 80mm lens on fx but zoom to 120mm with a dx camera. Sony flashes do this to but not canon. So claimed at dpreview.
Flash ( Speedlight) functionality with APS-C and Full frame cameras: Beginners Questions Forum: Digital Photography Review

[stevebrot]

Apparently different flash models work differently when auto zoom is used. A nikon sb600 always zooms to the lens size but the sb700 would zoom to 80mm with an 80mm lens on fx but zoom to 120mm with a dx camera. Sony flashes do this to but not canon. So claimed at dpreview.
Flash ( Speedlight) functionality with APS-C and Full frame cameras: Beginners Questions Forum: Digital Photography Review

I don't own the Pentax AF360/540FGZ or the version II of the same, but it is my understanding that they will do the same as the Nikon if an auto-focus lens is mounted. My Sigma EF610DG Super (Pentax flavor) will automatically zoom with AF lenses and automatically apply the crop factor (i.e. 35mm lens results in zoom set to 50mm for APS-C). Flash display distance scales are also automatically managed with AF lenses (semi-auto for A-series if flash zoom is manually set).


Steve

[swanlefitte]

I don't own the Pentax AF360/540FGZ or the version II of the same, but it is my understanding that they will do the same as the Nikon if an auto-focus lens is mounted.


Steve

I own the af360fgz but my mx won't work to check ff.
Prepost edit: i just need to check zoom on apsc. Do I get ff zoom?
Waiting for older cold batteries to charge...., surprise results. Very interesting.
First off it does zoom for ff equivalent. On apsc it with the sigma 18-35 and pentax 18-50 it adjusts as expected and displays 24mm zoom at the 18mm.
The 77mm and sigma 70, it displayed mostly 58mm but flipped to 85mm once in awhile.
With the 55-300 plm it wouldn't switch to anything other than 58mm.
With the da1.4x tc it didn't register the difference.
Any tests you can think of to understand this better?

[stevebrot]

The 77mm and sigma 70, it displayed mostly 58mm but flipped to 85mm once in awhile.
With the 55-300 plm it wouldn't switch to anything other than 58mm.
With the da1.4x tc it didn't register the difference.
Any tests you can think of to understand this better?

That sounds strange, though the 58mm part might be a clue.

According to the manual, auto zoom does not work in "Auto" flash mode, in that mode zoom must be manually set. Other than that, the manual seems to indicate that the zoom should automatically set on shutter half-press or when the meter is active. When setting zoom manually with an APS-C body, the longest setting is 58mm (equivalent to the 85mm longest setting for 35mm film). Pages 17-22 in the manual might be helpful.

Is your camera awake with meter on when 58mm is displayed?


Steve

[swanlefitte]

The camera is awake. I meant pttl auto zoom mode. Not 'a' mode. Since the sigma 18-35 in pttl auto zooms to 50mm and the 55-300plm zooms to 58 even at 300mm or at 58 from 55-300 with the 1.4xt it is strange.
I am set to bbf. But the 18-35 and 18-55 adjust as I zoom without a press I am not sure the half shutter means much.