Lower power setting = sharper focus?

[masguy 0]

I'm reading a book on underwater digital photography in which the author advises (and shows an example) that reducing the power setting on your strobe will actually result in sharper focus. I still can't get my head around this. Can someone explain (in layman's terms) why this is so?

[Alex_Mustard 0]

It is hard to tell what he/she is on about with seeing the whole passage.

 

All lenses are inherently sharpest at their middle apertures - sort of around F8-F11. Often what you pay for in an expensive lens is less fall off in sharpness away from this point. Most modern lenses are jolly good and such issues really aren't important these days.

 

I guess what the book is on about is that most macro stuff is shot up around F16-F22. By reducing the flash power you would be able to shoot at F8. Which would be more in the sweet spot of the lens. And therefore possibly slightly sharper. Also depending on the camera (sensor size etc) it might also reduced diffraction.

 

But these factors are very very minor. And 99 times or more you would benefit far more from more depth of field (shooting at F16-F22) than any theoretical extra sharpness at F8 in macro photography. It is not as if underwater photographers are all complaining of a lack of sharpness.

 

The main advantage of opening up the aperture and reducing depth of field is for subject isolation from the background. This can give the impression of the subject looking sharper as it stands out more.

 

But if this is what they are talking about it is total rubbish for real world shooting. And a very misleading for new photographers.

 

Anyway it is hard to tell what they are on about without a bit more detail.

 

Alex

[masguy 0]

Alex,

 

Thanks for the quick response. My source is “The Master Guide For Underwater Digital Photography†by Jack and Sue Drafahl (Amherst Media, 2005). On page 64 of that book, the authors state:

 

“At full power, most flash units are at 1/1,000 to 1/1,500 second. If you use one of the reduced-power methods mentioned above, the flash duration could be as high as 1/12,000 second when at 1/16 power. This increase in speed will result in a sharper image, especially when shooting at extreme macro magnifications.â€

 

There it is. They don’t say why this is, but they do have a picture comparison that seems to prove their point - it still makes my brain hurt trying to figure this out.

[jaykirk 0]

The sharper "focus" your author is referring to probably has more to do with flash duration than objective sharpness. Typically the larger the power setting of the flash, the longer the duration of the flash cycle.

 

For example if your flash unit at full power has a duration of 1/1000 of a second, lowering it to 1/2 of full power would cut your duration to about 1/2000 of a second (theoretically........)

 

The shorter the flash duration the more likely it will be that you stop any motion.

 

This will be more apparent with high magnification macro than at wide angle. With most macro work, even the fastest shutter speeds that your camera will sync with cannot compensate for small movements within the depth of field. In these cases, a slightly slower shutter speed combined with lower power flash will better compensate for these artifacts.

 

Hope this helps!

 

-Jason

[jaykirk 0]

masguy-

 

You caught me in the middle of a post. Hate it when that happens!!

 

When using flash as your primary light source, your camera shutter speed as little to do with stopping motion. This is all up to the flash duration. The shorter the duration the better the "freeze" effect. Since you typically get much shorter duration times with your flash at lower power, reducing power can help stop any motion.

 

This is why you see so many great photographs taken at very slow shutter speeds (<1/10/s). You'd think "how can they hold that camera so still??". Here the photographer is just using the slow shutter to allow more background light into the frame but is still relying on the fast flash duration to stop subject motion. Even thought the shutter speed might be 1/10 of a second, your real flash speed might be 1/10000.

 

-Jason

[Cerianthus 55]

But the essence is, if you would need a 1/10.000 s flash duration for that. Wouldnt a 1/1000 s duration be OK?

[Alex_Mustard 0]

“At full power, most flash units are at 1/1,000 to 1/1,500 second. If you use one of the reduced-power methods mentioned above, the flash duration could be as high as 1/12,000 second when at 1/16 power."

 

This part is probably fairly accurate.

 

This increase in speed will result in a sharper image, especially when shooting at extreme macro magnifications.â€

 

I do not believe that the difference in flash duration between 1/1000 of a second and 1/10000 second makes any difference to sharpness in real world underwater photography.

 

So generally I would say, don't worry about it. In fact I think the benefits of using a powerful flash, low ISO and a small aperture for macro would far out weigh the compromises needed to reduce flash power to benefit from shorter flash duration for extra sharpness.

 

Alex

[sgietler 1]

This part is probably fairly accurate.

I do not believe that the difference in flash duration between 1/1000 of a second and 1/10000 second makes any difference to sharpness in real world underwater photography.

 

So generally I would say, don't worry about it. In fact I think the benefits of using a powerful flash, low ISO and a small aperture for macro would far out weigh the compromises needed to reduce flash power to benefit from shorter flash duration for extra sharpness.

 

Alex

 

hey Alex, by low ISO, which ISO range were you referring you? I had read that images taken with iso 100, 200, and 400 by the nikon D80 and D200 will not have noticeable differences...and if someone has a weaker strobe, they might need to go to iso 200 or 400 to shoot with the smallest apertures...

 

would you consider ISO 200-400 a low ISO?

 

just curious,

 

scott

[masguy 0]

I wrote to the book's author and he was kind enough to reply. Here's what he had to say (which echos some of ehat has been said here). For those who have remarked that the difference is marginal, pick up the book and look at the difference - it's substantial. On to the author:

 

First the technique comes from the field of scientific photography. I learned it at Brooks Institute of Photography in 1968. It works much like shutter speeds. As you increase shutter speeds from 1/1000 second to 1/2000 and then 1/4000, the images become sharper because the time the image is on the sensor is shorter and there is less movement of the subject. The same is with flash, assuming that the background light (sunlight) is so low that it has no effect on the exposure (very dark or black). This is most common with macro images, where the sunlight is non-existent in the image. Since the only light striking the sensor is the light from the flash, the duration is much like a shutter speed. At full power it would be 1/1500 second, at half power 1/3000 second, and at 1/4 power 1/6000 second. Shooting at 1/4 has a very short duration where the light is striking the subject, and is therefore sharper than at full power. Not much but just a little. This technique is popular with nature photographers that shoot macro topside. They use large flash systems, and then use reduced power to keep their super macro images sharp.

 

 

 

Hope this helps

Jack Drafahl

[Paul Kay 65]

"I do not believe that the difference in flash duration between 1/1000 of a second and 1/10000 second makes any difference to sharpness in real world underwater photography."

 

Whilst I'd like to agree with you, I'm not entirely convinced that the effect of slower flash durations can't be seen! Here is a shot take a few weeks back, together with a detail. As you can see their is evidence of 'streaking' in some particulate matter in the water which indicates an illuminant which has operated for long enough to record the particle's motion. At first I assumed that this might be as a result of using the inbuilt spotting light, but it doesn't show up on other exposures like this, even with slightly longer exposures, and I'm doubtful if it would record such details given an exposure of 1.60s at f/16. I may be wrong though, so if anyone else has any suggestions I'd like to hear them. The common prawn was shot at slightly greater than 1:1 so the flash would have been kicking out a substantial percentage of its power (it was shot on TTL) and hence the duration was at the longer end of the scale (perhaps 1/1000s ??).

[Alex_Mustard 0]

Wow. Good example Paul. Those streaks are distracting. I guess there might be a real world benefit - interesting stuff.

 

Paul, I guess the tadpole effect is from the slow fall off of light after the main pop of the flash? If your flash gave constant power from start to finish of the flash then the streaks would be more bar like than tadpole like?

 

An underwater photographer can achieve a short flash duration by using a many strobes. If you used 4 strobes then all can be set at low power and therefore have shorter flash duration, while still maintaining a powerful overall flash power.

 

Also different brands of underwater strobe are known to have different flash durations. Inons are known for being short duration for example. Other strobes, like some of the European ones have longer flash durations. This might influence your strobe choice for macro if your were worried about this issue.

 

I always like the way that a seemingly straight forward question can lead to education all round.

 

Alex

[masguy 0]

I always like the way that a seemingly straight forward question can lead to education all round.

 

Alex

 

Glad to be of help grasshopper. I'll be in Borneo next week an I will try to do some side by side comparison shots.

[Paul Kay 65]

"Paul, I guess the tadpole effect is from the slow fall off of light after the main pop of the flash? If your flash gave constant power from start to finish of the flash then the streaks would be more bar like than tadpole like?"

 

I'm still intrigued by this - it only seems to happen at very close distances!!! The only examples I can find are at greater than 1:1 which has me entirely puzzled. Not entirely sure on this side of my flash theory but I doubt that a flash tube would have a 'lower, after burn effect'. I can see uses for the effect - to illustrate swirls in the water and wonder if it could be used creatively at times. I think a bit of experimentation is in order.

[PRC 2]

hey Alex, by low ISO, which ISO range were you referring you? I had read that images taken with iso 100, 200, and 400 by the nikon D80 and D200 will not have noticeable differences...

 

Not Alex but..

 

Nope, On the D200 increased noise is noticeable as soon as you jack the ISO past 100, which is the lowest available setting on these units.

 

As the D80 has (I think) the same sensor I would expect it to be similar to the D200.

 

On a recent trip I used ISO 160 for some shots, and I can tell on the screen.

 

The D70 has a lowest ISO of 200 and the noise level at this setting seems very similar to the D200 at ISO 100. Different sensor technology.

 

Personally I remain envious of the Canon range here - the noise on their sensors is noticeably lower at higher ISO than the Nikon.

 

Paul C

[herbko 0]

Here's some old data on the flash duration of a few underwater strobes.

 

http://wetpixel.com/i.php/full/inon-d-2000...al-explanation/

 

The Inon D-2000 has a significant advantage over the others. This came out before the Z-240. My guess is that the brighter Z-240 will be a little slower to deliver full power.

[Jez_Tryner 0]

Also different brands of underwater strobe are known to have different flash durations. Inons are known for being short duration for example. Other strobes, like some of the European ones have longer flash durations. This might influence your strobe choice for macro if your were worried about this issue.

 

Thankx Alex ,you have unwittingly answered a question of mine. i am going to buy 2 x z-240's to replace my c'n'c dx-90's for a few reasons ,power, recycle time and size. but i wondered about the speed of flash firing.

i have been reading some of your other posts and some of your shots with d-70 are at 1/1000th , i can not get anywhere near there. you did mention an electronic shutter which i beleive will be the reason but with my canon 10d if i take the shutter speed higher than 1/250 i start to get a straight line through my shot, the higher i take the shutter speed the more the line moves across my shot so at 1/600th i have an exact flash/no flash effect on my shot, it's very cool and an interesting demonstration of the effects of flash on colour all in the same shot but doesn't help me with my high end shutter speeds, do you think it's the mirror coming down? and would a faster flash fire speed enable me to use higher shutter speeds?

i was going to post the shot, flash/ no flash as it's very cool but i don't have it with with me

cheers

Jez

[bvanant 195]

Thankx Alex ,you have unwittingly answered a question of mine. i am going to buy 2 x z-240's to replace my c'n'c dx-90's for a few reasons ,power, recycle time and size. but i wondered about the speed of flash firing.

i have been reading some of your other posts and some of your shots with d-70 are at 1/1000th , i can not get anywhere near there. you did mention an electronic shutter which i beleive will be the reason but with my canon 10d if i take the shutter speed higher than 1/250 i start to get a straight line through my shot, the higher i take the shutter speed the more the line moves across my shot so at 1/600th i have an exact flash/no flash effect on my shot, it's very cool and an interesting demonstration of the effects of flash on colour all in the same shot but doesn't help me with my high end shutter speeds, do you think it's the mirror coming down? and would a faster flash fire speed enable me to use higher shutter speeds?

i was going to post the shot, flash/ no flash as it's very cool but i don't have it with with me

cheers

Jez

The mechanical shutter for the 10D is your limitation here. The Nikon electronic shutter allows for much faster sync speeds than the 10D or even the newer Canon bodies like the 5D.