Freezing movement: fastest sync speed of RZ67?
I'm having problems with getting the sharpness I want from a moving subject. I'll have my model in the studio, simple two-light set-up, she's not doing anything particularly action packed, but still if she moves her arm I get blur. This was using Velvia 50, with a normal 90mm lens, mirror lockup...stars aligned..am I not using a fast enough shutter speed? At what point does the RZ not sync anymore?
On this thread we get 1/400 max sync speed: http://www.apug.org/forums/forum51/7...peed-info.html
Will that freeze arms and things tossed into the air properly (which may be a feature of the shots)? Sorry if this is an elementary question, I just haven't
used a moving human subject in a while, I need it to be blur-free for what I'm doing and if there's a better way to get that result I'd love to know about it.
I assume you are using strobes? If you are still getting blur, you could try turning the strobes down, which might shorten flash duration. Adjust f-stop accordingly.
Leaf shutter lenses generally synch at any speed, 1/400th should work fine.
Yes, using strobes, couple of Style 600rx Elinchroms. How much of a difference would it make to shorten the flash duration? Are we talking
minute fractions of a second?
The Style 600Rx is claimed to have a flash duration of 1/2050 sec on full power and this would be even faster as the power is reduced. In a studio environment using strobes the shutter speed normally plays no part in the exposure because the flash should overpower any ambient light and its duration effectively becomes the exposure time. If you are getting subject blur I would suggest that either your studio has high ambient light levels eg a large window or skylight or your modelling lights are too bright. Your blur is possibly the result of a secondary available light exposure at the shutter speed set on the camera. OzJohn
Originally Posted by Holly
Last edited by OzJohn; 05-15-2012 at 12:36 AM. Click to view previous post history.
Reason: correct terminology
1/400 is the max speed of the camera - are you set at that speed or something slower? How much ambient light do you have? If there's a lot, you might get visible blur from the ambient contribution regardless of the flash duration. Keep in mind also that the 1/400 setting is 1/400 worth of light, taking into account the non-instant opening and closing motion of the shutter. The total duration for which there is any exposure is closer to 1/250.
Assuming you're at 1/400 and your ambient is very low (i.e. requiring at least 1/15 exposure at your chosen aperture therefore 4+ stops underexposed), the limiting factor to the exposure is your flash. Cheaper flashes (higher-impedance capacitors that have a longer discharge) can have an exposure as long as 1/500 so not really much improvement over the shutter speed. Better ones will be about 1/2000 to 1/10000 depending on power settings. You should be able to look up the flash duration for your strobes but be careful: it can be quoted as a t.1 (10% extinction) or t.5 (50% extinction) time and clearly the lower extinction value takes more time. A flash rated at 1/1000 t.1 is much faster than a flash rated at 1/1000 t.5.
Turning down the flash power will almost always shorten the flash duration, approximately according to power. Take two stops off, the flash should go about 4x shorter in duration, but there are some funny lumps in the speed/power graph where capacitors are switched in/out to control power instead of adjusting voltage. It all depends on the flash model and at some point, you will reach a minimum duration, which may or may not be at minimum power.
Tossing an apple lightly in the air means an arm speed of about 2m/s, which is 5mm in 1/400s, 10mm in 1/200s. If you have any ambient contribution, you can expect to see plenty of blur! Say your flash is a moderately decent one with 1/2000 duration, you're still looking at 1mm of motion blur in the exposure. If you want to halt extreme speeds, the common approach is to use a large hotshoe speedlight set at its minimum power setting (1/32 or so), which will give you exposures down around the 1/16000 mark for 0.1mm of blur. You obviously don't get much light like that though.
Beyond that, you're talking about building a custom high-voltage, low-capacitance flash circuit. Very dangerous stuff but you can probably hit 1/50,000s, at which point you can run into short-exposure reciprocity failure on some films.
Edit: these are the flashes you were asking about transformers for, right? I take it you have them all hooked up and working nicely (other than motion blur) now then?
Last edited by polyglot; 05-15-2012 at 01:19 AM. Click to view previous post history.
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Oops, it's worse. Tossing an apple 1m in the air requires a launch velocity of 4.4m/s, so it's 2x as bad. 1/2000 will leave you with >2mm of blur.
By kinetic/potential energy exchange:
E = mgh = 0.5 mV^2
V = sqrt(2gh); g=9.8m/s^2, h=1m, V=4.4m/s
Reduce flash power and keep the ambient light at least 4 stops under the strobes, and that should address the problem. The sync speed isn't important, unless you have a lot of ambient light relative to the strobes. If you need a lot of strobe power at short duration, then the solution is to divide the power among more strobe heads or use dual-tube heads, so you might have two heads at half power and about half the duration of one head at full power.
Another thought... if motion blur is the only problem, try to time the shot when things are obviously in-flight but actually stationary, i.e. at the top of their path.
Say someone tosses an apple; by the time the apple reaches its highest point, their arm will be pretty much stationary and the apple will too. Press the shutter at that point and you will get practically no blur. The tradeoff is that you lose the muscular tension in the actor and the pose is relaxed/spent instead of tense/powerful. The flying apple and extended arm will imply the motion but it won't (IMHO) be as visceral as capturing the model in the middle of throwing.
Oh dear god, I knew this would prompt horrible mathematical things! lol
Originally Posted by polyglot
So yes, it's the same lights I got the trannies for (as they are now affectionately known). They work fine!! Really happy with them! They're bloody heavy but they were the best investment, I've had no worries at all.
I'm working in a studio with one big window but whenever I've shot there, it's either been after dark so no daylight
coming in, or I've had a pretty thick UV-blocking-backed curtain drawn. Only other ambient light is the crappy fluorescent
strip lighting to the room itself.
I think for the arm-blur session I was shooting on 1/60. I've just done a mini test of 1/400 with a moving target (a glove
spinning around at a medium speed hanging from a string - don't ask) and can see when that's dev'd how it worked.
Left the manual at the studio, but as OzJohn said, the Style 600rx should be around 1/2050, so that's a decent flash.
I've noticed this blurring thing happening in a totally different studio with no windows whatsoever and even crappier
dimmer fluoro lighting - and the strobes were Elinchrom 1200s in there.
If I lower the flash output, how can I keep my big depth of field though? I'm aiming to have every single hair and molecule
of detail in focus so I've been aiming for f16 + (going on the idea that the smaller the aperture, the bigger the DOF). Although
isn't it true that for every lens, the sharpest aperture is going to be about half the maximum aperture number? i.e. an f32 lens
will be sharpest at f16.
Someone like Harold Edgerton would have had a specialised flash then yeah?
I always bring the horrible mathematical!
Shooting at 1/60 could well be a problem as the lighting from the fluorescents might show up. Unlikely at ISO50 f/16, but possible. I guess you'll know when you get the 1/400 test back.
To reduce power but maintain exposure & DOF, you can move the light closer to the subject (can cause issues with uneven light but twice as close gives you about +2 stops) and/or use a faster film. I'd suggest (separately from your love-affair-with-Velvia question) trying some Portra 400. Especially if you're doing a hybrid (scanned) process, you can add in plenty of saturation later. It's not as fine as RVP but it's still very good in 6x7. You can also try Provia 400X - a bit pricey but it has the saturated contrasty chrome look.
Sharpest aperture on most RZ lenses will be about f/11 or f/16. f/11 might give you more resolution but obviously slightly less DOF. Something else you can explore is lower magnifications, i.e. including more in your frame and making the subject smaller by stepping the camera further back and/or using a shorter lens. That will increase your DOF without needing more light intensity.
If you want stationary-looking bullet photos, you need very custom flash hardware, big apertures and fast film.