[mcgregni]

Some clarification perhaps required here .... Ambient exposure is controlled by Aperture, ISO and shutter 'speed' .... Flash exposure is controlled by Aperture, ISO, flash power, flash head zoom and flash to subject distance.



So, if all other factors remain equal, then an aperture adjustment will change both ambient and flash exposures, as will an ISO change. A shutter 'speed' change will only affect ambient exposure.



Remember that if using an auto-exposure camera mode (eg P), then all else will not remain equal (auto adjustments will occur to retain the meter exposure value), and in a flash auto mode (eg P-TTL) then all else will not remain equal either as the flash power will adjust in response to maintain the meter flash exposure value.



It is only in M exposure mode and M flash mode that all else will remain equal and it is easy to see the immediate effect of each adjustment.



The reason that shutter'speed' will not affect flash exposure is because the flash light burst is extremely quick .... Perhaps 1/10,000 sec, and relatively very powerful compared to the ambient.(on the things it strikes) ... And it occurs within a shutter time of anything up to 1/180th sec



So it's affect on the ambient exposure is almost nothing because time is not a factor in how much impact it has on the sensor (there's just not enough time). The hole through which the flash must pass (the aperture opening), will of course have an impact on how much light reaches the sensor, regardless of how briefly the light shines for.

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Last edited by mcgregni; 07-18-2015 at 03:36 PM.

[adwb]

You've been reading threads about this subject for years, and have not read a valid reason for people wanting a faster sync speed? Honestly?

That's not what I said what I said was I have never see a decent reason to worry aboy 2/3 of a stop.

I understand the 1/180 or even 1/250 restrictions that's why I bought another brand just to get the electronic and therefore high speed shutter sync because I need it and can't afford or justify the cost of a body that accepts focal shutter lenses but the 180 vs 250 is a pointless desire.

[clackers]

. Aperture controls flash exposure, shutter controls ambient exposure and iso effects both. I don't know the exact science behind it.

Then you need to read Mcgregni's post, because what you said is not right.

[mcgregni]

Well, I try not to claim exclusive rights to 'being right' .... That can result in some egg on face moments! I think the previous quote was just missing the bit about aperture also affecting ambient ... Of course it is part of the ' exposure triangle' .



When flash is active though, in some automatic configurations it is common for ambient light to be almost wiped out ... Unless setting 'Slow-Sync' in dim conditions then that is what can happen.



This could easily make it appear that aperture is not affecting ambient. It is, but in those conditions it is just too tiny!



As I said earlier, even with Slow -Sync, automatic exposure control will counteract aperture adjustments, again making it appear to have no effect.

[BrianR]

When flash is active though, in some automatic configurations it is common for ambient light to be almost wiped out ... Unless setting 'Slow-Sync' in dim conditions then that is what can happen.

This could easily make it appear that aperture is not affecting ambient. It is, but in those conditions it is just too tiny!

As I said earlier, even with Slow -Sync, automatic exposure control will counteract aperture adjustments, again making it appear to have no effect.

This is what I'm wondering, if some sort of auto type settings were countering things to make it appear like aperture was only affecting the flash.

It's funny, but I've seen several people make the same sort of claims that shutter is for ambient and aperture is for flash, so I'm kinda wondering where it's coming from. There may be a confusing reference somewhere on the interwebs or maybe people are jumping to conclusions from a specific example of how a photographer might chooses settings (i.e. set flash power, then use aperture to adjust, then use shutter as your adjustment for the ambient, then not realizing how the steps are interconnected).

[mcgregni]

You're right that it can become confusing, once flash is activated, .... Not obvious exactly what mode (on the camera AND flash) is influencing exactly what aspect of the brightness. Manual everything brings more certainty, but then you still need to know the theory and specific settings to take control of .



I've been thinking (and writing) a lot about all of these mode interactions just recently. Keep an eye on the homepage .... Soon there will be a whole lot more information available .....

[savoche]

I've been thinking (and writing) a lot about all of these mode interactions just recently. Keep an eye on the homepage .... Soon there will be a whole lot more information available .....

Yep, it's up now. Looks to be very comprehensive.

Thanks for sharing this, Nigel!

[mcgregni]

You're welcome some bed-time reading for Pentaxians ! There's a fair bit of info within the guide about exactly what has been questioned here ...about the max sync speed and the controls for independently balancing ambient and flash exposures.

Sorry, I don't know why we can't have 1/250th .... I can only assume it's related to the specific hardware of the shutter mechanism and Pentax are committed to it.

[clackers]

Sorry, I don't know why we can't have 1/250th .... I can only assume it's related to the specific hardware of the shutter mechanism and Pentax are committed to it.

As I understood it from other posts, shutters are purchased from only a couple of vendors and this one is particularly robust/durable.

[mcgregni]

Hmmm ... In order to provide a 1/250th sec max sync then the blades would have to move faster. I wonder if this takes its toll on durability? We have very generous shutter click ratings on Pentax DSLRs .. Even my 'old' K7 is rated to 100,000.

I wonder if comparable models from the competition that offer 1/250th have the same durability and reliability ... We don't hear of many Pentax shutters breaking before they reach their max rating, do we?

I think I would happily live with 1/180th if it prevented a repair bill ....

[Timd]

1/180 sync speed would also be quieter than 1/250 sync speed. Pentax cameras since K5 especially are noticeably quiet.

[Class A]

A K-5 (II)(s) with its ISO 80 setting, that beats most camera's minimum ISO 100 setting, shrinks the pretty small .47 stop difference between 1/250s and 1/180s down to a .15 stop difference (which is all but negligible).

I'd rather take the ISO 80 than a sync-speed of 1/250s.

[mcgregni]

I agree; it's not really a deal breaker for me ....certainly not enough on its own to consider another camera system . As you say ClassA, exposure wise there's not much in it .... Does the extra 1/70th sec have significant action freezing effect ?? A bit of fill flash would have some movement freezing effect anyway, on foreground subject material .

[BrianR]

I agree; it's not really a deal breaker for me ....certainly not enough on its own to consider another camera system . As you say ClassA, exposure wise there's not much in it .... Does the extra 1/70th sec have significant action freezing effect ?? A bit of fill flash would have some movement freezing effect anyway, on foreground subject material .

'tis not 1/70th faster (which is about 0.014 seconds), it's 1/180-1/250 =7/450 seconds (or about 0.001555... seconds). The 'half a stop' train of thought is probably the most meaningful measurement.

For me it's mostly working outdoors combining sunlight with flashes where the aperture and iso end up with little leeway to stay under the sync speed. It's much better now that I have a k5iis with iso 80 compared to my old k100d which bottomed out at iso200 (as Class A points out, iso 80 is very handy to have!). I also usually have the luxury of having flash power to spare, so dropping the iso (or nd filters) can work for me (but this isn't the case for everyone). This doesn't mean I wouldn't make use of 1/1250th if it were available though.

edit- the extra half-stop of shutter would also give an extra half stop of hand hold-ability for when I'm being lazy and not using a tripod in the above situations. There are a few things I'd happily give up for this extra half stop if I had the option (like the pop-up flash)

[mcgregni]

You're right, I really should have kept to 'stops' and not put my instant arithmetic to the test ! I can see also, at telephoto lengths, that 1/250th will offer that little bit extra 'hand holdability' .

Without a flashgun on and set for HSS then it is a practical limitation, for sure.