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I'm using an old Minolta Auto 25 flash off camera wireless to my FUJI XT2. There is no manual power setting switch, only an ISO setting dial. I do not want to use auto mode, but I also do not want to increase flash to subject distance to lower light intensity. So, if I set the flash ISO setting dial to 800, will the flash emit less light/power than if it was set to the same ISO setting as the camera, in this case, say 100?

In other words, if the camera ISO is set at 800, it makes the sensor more sensitive to light. So if I set ISO on the flash to 800 and the camera to 100, is it fooling the flash to emit less light believing it is providing enough light to the camera if the camera setting was 800?

Minolta Auto 25 front Minolta Auto 25 back

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After glancing through a manual I found online, I would guess that the backside dial is not connected to anything electrical, and only works as a mechanical calculator for the unit's two settings:

  • Auto, for constant measured light at the sensor
  • Manual, for constant (full) output of the flash

But you have a digital camera, you can try it anyway!

If you only want a darker photo, use a smaller aperture or lower ISO.

If you want to balance the flash with existing light, or use a larger aperture, you can waste the flash output somehow:

  • Bounce the flash from the ceiling or walls
  • Make a DIY softbox or diffuser from a white plastic container
  • (Why don't you want to take the flash further from the subject? You want to hand hold both the camera and the flash?)
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The Minolta Auto 25 is an Auto Thyristor flash with a sensor on the front that can vary the flash output in auto mode.

The light output will not change by adjusting the ISO wheel. It is only there for a reference to help you set the aperture for auto or manual use.

At ISO 100, f/5.6 should be used for Auto flash and the sensor will adjust the power to maintain the proper f/5.6 exposure anywhere from 15’ to as close as 2.3’

Changing the ISO to 400 would require you to set an aperture of f/11 and again the auto range would be from 15’ to 2.3’, but the actual light output would be the same regardless of what is set on the ISO wheel.

I assume you are finding the flash output too high for the way you are positioning the flash near your subject. Since you are using this flash off camera, I would suggest using Auto mode so that the flash will automatically reduce its power at close range. If this is still too bright for you, just adjust the aperture or ISO on the camera for the desired result.

This is not a very powerful flash and with a bit of trial and error, it should be very easy to vary the camera exposure for the desired results.

enter image description here

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The wheel on the back of the flash is not connected in any way to the flash's internal controls. It is there as a mechanical calculator to allow you to easily calculate what Aperture, film speed, and shooting distance will work.

The "Auto" range on the dial indicates the near and far limits when the flash is set to "auto thyristor" mode via the switch on the front of the flash. Notice that when the switch is set to "Auto" on the front, the exposed part under the switch's slide is the same color as the "Auto" range on the calculator.

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In the case of ISO 100, the flash can throttle the power down at f/5.6 anywhere from 15 feet (more power) to 2.3 feet (minimum power).

With a digital camera, you can adjust the camera's ISO setting to whatever you selected on the dial calculator, or vice versa.

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