How to use the Flash Zoom


sebas77

Link Posted 09/04/2017 - 23:08
Hi,

stupid question, is the flash zoom supposed to match the lens focal when it's on camera? Is the flash actually supposed to read the current focal length and set it automatically? I think this is what it usually does, however on my Metz AF-2 it doesn't always work, I guess it depends by the lens. Is it supposed to work with the series A lenses?

pentaxian450

Link Posted 10/04/2017 - 02:08
With APS-C Pentax cameras, the flash focal length shown on the flash head is the value for a FF camera because the zoom on the flash is calibrated with full frame in mind. In practice, although there seems to be a discrepancy between the flash and lens focal length, the illumination of the flash will match the lens focal length.
Yves (another one of those crazy Canucks)

sebas77

Link Posted 10/04/2017 - 08:07
In my case it's on a K1, so theoretically the flash zoom should always match the focal length? Even if I use an A series lens?

McGregNi

Link Posted 10/04/2017 - 12:32
Automatic flash head zoom is only compatible with autofocus lenses, so you would have to manually set the zoom with an A type lens.

As Yves said, for many flashes the display will indicate the FF figure. With Pentax we have more sophistication in that the AF-360/540 FGZ can automatically detect the camera sensor format (aps-c, 35mm, 645 etc). In the case of the older 'mk1' models there is a manual override for this setting also.

This is really only directly related to the actual field of view when the flash is on the camera and pointing straight ahead. For off camera and bounced flash situations then the flash head zoom setting is fairly unrelated to the actual lens focal length, and is mostly a creative choice. Even on camera and directly ahead then we often don't need to illuminate the entire frame, as our subject may only take up a small area, so a longer setting on the flash zoom may be more efficient (it will use less power for the same exposure as the light is concentrated into a tighter beam).

If you had the camera format incorrectly set on a flash you risk wasting power though .....say you were using a 55mm lens on an apps-c camera; if the flash zoom was set to, say 50mm, and the format was FF, then a lot of the light would be wasted around the outside of the image frame. It would be necessary to manually zoom the flash head in to, say 80 or 85mm to match the actual field of view being both 'seen' by the sensor and illuminated by the flash.

Automatic format detection has the advantage that this conversion is carried out and even though the flash head will be at the position for illuminating a FF field of view of 85mm, the actual display on the flash screen will show 50 or 55, so relating neatly to the actual lens focal length.

For a fuller rundown see Section 8 in my guide, 'Flash Head Zoom and Camera Format', pages 46-49 (downloadable from the link in my signature here ...)
My Guides to the Pentax Digital Camera Flash Lighting System : Download here from the PentaxForums Homepage Article .... link
Pentax K7 with BG-4 Grip / Samyang 14mm f2.8 ED AS IF UMC / DA18-55mm f3.5-5.6 AL WR / SMC A28mm f2.8 / D FA 28-105mm / SMC F35-70 f3.5-4.5 / SMC A50mm f1.7 / Tamron AF70-300mm f4-5.6 Di LD macro / SMC M75-150mm f4.0 / Tamron Adaptall (CT-135) 135mm f2.8 / Asahi Takumar-A 2X tele-converter / Pentax AF-540FGZ (I & II) Flashes / Cactus RF60/X Flashes & V6/V6II Transceiver
Last Edited by McGregNi on 10/04/2017 - 12:43

Helpful

sebas77

Link Posted 10/04/2017 - 12:50
Thank you, yes it makes sense only on camera, I specified that in my original post because I understand it's obvious.

I actually never thought, until few days ago, how the zoom flash can affect the light field of view, which is very interesting even for off camera lighting setup. Thank you a lot McGregNi, you are very helpful, and from now on I will bother you to know more about this stuff

P.S.: on the K1 I don't have FF/CF problems hehe...anyway the metz has also the "zoom size" option that does the conversion.
Last Edited by sebas77 on 10/04/2017 - 13:08

McGregNi

Link Posted 10/04/2017 - 13:02
Yeah, I imagine that the clever automatic format detection stuff is the preserve of proprietary camera brand systems with their own equipment. It's interesting that the original AF-540 FGZ model uses the 'Digital' label to describe the aps-c format setting, because when the flash was designed aps-c was the only digital format in the range !

I guess nowadays it's quite possible that a Pentax photographer might mix and match their flash unit between a K3, K1 and 645z! Then the automatic format detection would save some time and effort. For most of us it's going to be a set it forget it type of thing.

Glad to help, ask away anytime.
My Guides to the Pentax Digital Camera Flash Lighting System : Download here from the PentaxForums Homepage Article .... link
Pentax K7 with BG-4 Grip / Samyang 14mm f2.8 ED AS IF UMC / DA18-55mm f3.5-5.6 AL WR / SMC A28mm f2.8 / D FA 28-105mm / SMC F35-70 f3.5-4.5 / SMC A50mm f1.7 / Tamron AF70-300mm f4-5.6 Di LD macro / SMC M75-150mm f4.0 / Tamron Adaptall (CT-135) 135mm f2.8 / Asahi Takumar-A 2X tele-converter / Pentax AF-540FGZ (I & II) Flashes / Cactus RF60/X Flashes & V6/V6II Transceiver

sebas77

Link Posted 10/04/2017 - 13:53
the guide you wrote is incredible. I will upload it to my kindle and take my time to read it.
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