First Flash Second Question

[george013]

Well GN calculations can be used to determine exposure for direct flash for a given power level and distance.

Dave

And if you recalculate the guide number for the minimum flash power, I think 1/64, then you know the minimum distance for 100 iso. Guidenumber = distance in meter*f-number. At 100 iso mostly. You've to check the specs of the flash.

George

[dje]

And if you recalculate the guide number for the minimum flash power, I think 1/64, then you know the minimum distance for 100 iso. Guidenumber = distance in meter*f-number. At 100 iso mostly. You've to check the specs of the flash.

George

George I was a bit rushed before but I have tried doing what you suggest and also had a closer look at handbooks. As an example, take the SB-700. It has GN of 28(m) at ISO100. For an aperture of f/32 and a power level of 1/128 (which seems to be the minimal manual setting available), the distance for correct exposure is about 80mm (direct flash). However the smallest minimum distance specified in the handbook (for iTTL) is 600mm. I'm afraid I can't rationalise this, I'm just not sure where the 600mm comes from. Grahame's test reported in post 20 seem to support a much closer usable distance than 600mm.

The SB700 handbook contains a chart which indicates that for ISO 100, the minimum distance rises above 600mm only for wide apertures in the range 2.8 to 1.4. In this range of aperture for a 105mm zoom head position, the minimum distance drops from 1.7m to 900mm as the aperture changes from 1.4 to 2.8 and appears to follow the general trend predicted by the GN calculations. It is almost as though an arbitrary cut-off point of 600mm has been applied.

Go figure!

Dave

[george013]

George I was a bit rushed before but I have tried doing what you suggest and also had a closer look at handbooks. As an example, take the SB-700. It has GN of 28(m) at ISO100. For an aperture of f/32 and a power level of 1/128 (which seems to be the minimal manual setting available), the distance for correct exposure is about 80mm (direct flash). However the smallest minimum distance specified in the handbook (for iTTL) is 600mm. I'm afraid I can't rationalise this, I'm just not sure where the 600mm comes from. Grahame's test reported in post 20 seem to support a much closer usable distance than 600mm.

The SB700 handbook contains a chart which indicates that for ISO 100, the minimum distance rises above 600mm only for wide apertures in the range 2.8 to 1.4. In this range of aperture for a 105mm zoom head position, the minimum distance drops from 1.7m to 900mm as the aperture changes from 1.4 to 2.8 and appears to follow the general trend predicted by the GN calculations. It is almost as though an arbitrary cut-off point of 600mm has been applied.

Go figure!

Dave

To tell the truth, I'm not a flash man.
I've the SB-700 too. With the slider on the left hand you can chooise ttl,m or gn.
With ttl you let the combi flash/camera/lens decide the flash power.
With m you decide what the flashpower will be.
With gn you select a distance and the flash power is changed according to that distance between 0.3 and 20m.
I think ttl needs a bigger minimal distance for the preflash to work. There's no preflash in m and gn.
With the flash on the camera it shows ttl-bl in ttl mode. When off the camera it shows only ttl. The ttl-bl needs the info of the D-lens. Maybe Art won't see ttl-bl on his flash with his lenses.
Things have changed with bl. I remember my first Nikon the D80 and the SB600. Bl was only possible with the flash head pointed forwards. And if you think about it, it is logical. In bl the flash takes in account the distance given by the lens so the flash needed the same direction as the lens. Bouncing would mean the light travels much more before hitting the subject. So I don't know what bl means now if the head is positioned in another direction as the lens.
I hope my writing does make sense. I still have to read the manual.

But what I'm sure of is that shooting with flash is a lot of experimenting and getting experience with it.

George

[Manfred M]

I suspect there might be another factor at play regarding the minimum distance recommendations.

All of these modern flashes have a motor driven flash tube that moves backwards and forwards to control the light spread. The position is based on the focal length of the lens (and of course the crop factor of the camera). Longer focal lengths mean the flash tube is further back and the reflector shoots out a narrower beam while getting closer moves the flash tube forward, almost against the plastic diffuser and this will result in a much wider beam. My SB-900 has an auxiliary flip out diffuser for really wide angle shots.

Get the flash too close to the subject and the light spread would be too narrow to illuminate the subject evenly. The unfortunate part of the manuals is that they provide a number, but there is no explanation as to the basis of the data, so the users do not understand the "why" of this flash parameter.

[Manfred M]

To tell the truth, I'm not a flash man.

I thought as much.

Preflash is pretty versatile and works well even in off-camera mode and with more than one flash in use. It works well when the flash head is turned and light is bounced, etc. With iTTL, it sees what the camera's metering system sees, i.e. what the photographer sees through the viewfinder. I would be very surprised if it is a preflash issue and suspect something else is in play (see #24).

What I don't know about is the use of non-CPU lenses as until recently, the only one I owned was an 8mm fish-eye and the flash was definitely useless there. I was recently given a three old Nikon F mount lenses with a Nikkormat film camera. I have all of those focal lengths on newer lenses so have not tried the non-CPU lenses on my D810 yet. When I get some time, I will do some testing with flash.

[pschlute]

This thread prompted me to have a look in the manual for my own flash units. It helpfully gives all the guide numbers and the formula for working out min/max effective range. At the end it says "if your calculated minimum is less that 0.7 metre then use 0.7 metre instead.

[Stagecoach]

So what this thread raised was the question as to where this figure for "minimum flash working distance/range" comes from in the flash specs when we all know that we can use TTL flash far closer to a subject with excellent auto control than 'suggested' in the range figures given by the makers.

I believe the manufactures base the figure, often 0.6 m as the flash to subject distance where you will achieve an 'acceptably' even illuminated and exposed subject (think flat surface) within the FOV of the lens at the aperture and ISOs given in the table, when the flash is mounted on the camera pointing directly ahead.

I came across this by chance in the SB-600 manual which gave me the clue;
.................................................. .................................................. ........
When shooting subjects closer than 0.6 m (2 ft.)
With the SB-600 attached to the camera, sufficient illumination of the subject
cannot be obtained. In this case, use the SB-600 off-camera by attaching the
optional TTL Remote Cord as shown below.
.................................................. .................................................. .........

It then showed a picture of exactly the same setup as shown in my earlier post, the flash off camera and close to the subject.

The below diagram, to scale (sensor, flash head, angles and distance) shows an 85mm lens and flash (at 85mm zoom) projecting a beam at a plane 600mm distance. I suspect the actual flash beam angle is greater than the lens 16 deg FOV and is more like the magenta line and is determined at that which will produce a reasonably even light coverage.



You can see from the above diagram that if that 600mm subject distance plane was to move nearer the camera/flash the illumination of the lens FOV will become less even and less lit.

There are also some other clues regarding near distance shooting (not macro close) such as, angle the flash down (not possible with some flashes when camera mounted), and use a wider flash zoom angle than the lens.

[george013]

I thought as much.

Preflash is pretty versatile and works well even in off-camera mode and with more than one flash in use. It works well when the flash head is turned and light is bounced, etc. With iTTL, it sees what the camera's metering system sees, i.e. what the photographer sees through the viewfinder. I would be very surprised if it is a preflash issue and suspect something else is in play (see #24).

What I don't know about is the use of non-CPU lenses as until recently, the only one I owned was an 8mm fish-eye and the flash was definitely useless there. I was recently given a three old Nikon F mount lenses with a Nikkormat film camera. I have all of those focal lengths on newer lenses so have not tried the non-CPU lenses on my D810 yet. When I get some time, I will do some testing with flash.

I was just thinking in the same direction as Graham sketched. With the addition of the preflash.
If the camera doesn't have the info about the aperture, lens info, then the flash can't work correctly. And your camera can't control the aperture. I just tried with an inverse ring, per definition non cpu, the camera can't control the aperture. But the flash is still in ttl and even in ttl-bl.
Read post 22, the one I answered.

For an aperture of f/32 and a power level of 1/128 (which seems to be the minimal manual setting available), the distance for correct exposure is about 80mm (direct flash). However the smallest minimum distance specified in the handbook (for iTTL) is 600mm

I didn't recalculate it but I think it's practical nearly impossible to make use of a preflash at a distance of 80mm from the head. And then have that light being collected by the lens. Off course in a off-camera position that would be possible.

@Dave,
Rereading your post I think that 80mm is based on f2.8.
For the minimal distance you've to take in account the focal length of the flash. Nominal is 35mm. I think that what you call a cut-off is related to that angle. And on camera.


George

[Stagecoach]

I didn't recalculate it but think it's practical nearly impossible to make use of a preflash at a distance of 80mm from the head. And then have that light being collected by the lens.

To further diagnose you could set your camera/flash up as per the above diagram with the subject say 200/300mm from the flash.
Test to see that with the flash in manual varying its power up to full there is enough light on the subject to provide a good exposure.

Note, that if this test is done in TTL mode due to the lower reflected light returning to the metering system from the preflash the camera control system would attempt to compensate with greater light from the flash i.e longer duration flash.

[dje]

Grahame it seems to me that the 0.6m figure is a sort of "rule of thumb" rather than a figure based on precise calculations because it is used for a range of fstop vales and flash beam widths. It is probably to guard against the sort of illumination issues you have indicated and also the effect of uneven illumination on pre-flash iTTL metering. As well as the illumination problem you highlighted caused by lateral displacement of the flash off the lens axis, there is a distinct possibility that the radiation pattern of the flash close in has a few lobes off to the side of the main beam which would add to the uneven illumination problem.

George I believe my calculations are correct. Here is some more detail

GN full power for SD700 = 28 for ISO100
GN min power (1/128) = 2.475 (28/SQRT128)
Distance for correct exposure at min pwr and fstop 32= 2.475/32 =0.077m which I rounded to 0.08m (80mm)

Dave

[Stagecoach]

Grahame it seems to me that the 0.6m figure is a sort of "rule of thumb" rather than a figure based on precise calculations because it is used for a range of fstop vales and flash beam widths. It is probably to guard against the sort of illumination issues you have indicated and also the effect of uneven illumination on pre-flash iTTL metering. As well as the illumination problem you highlighted caused by lateral displacement of the flash off the lens axis, there is a distinct possibility that the radiation pattern of the flash close in has a few lobes off to the side of the main beam which would add to the uneven illumination problem.

I agree Dave, in fact rather than a "rule of thumb" I would say this 0.6m that's used in the spec charts for TTL capabilities is "a figure totally not applicable to close/macro work", and this thread was about using a macro lens and flash.

In addition I would go as far to say that the info in my flash manual that states ...........

.................................................. .................................................. ........
When shooting subjects closer than 0.6 m (2 ft.)
With the SB-600 attached to the camera, sufficient illumination of the subject
cannot be obtained. In this case, use the SB-600 off-camera by attaching the
optional TTL Remote Cord as shown below.
.................................................. .................................................. .........

is rubbish, because sufficient illumination giving good exposure is obtained at distances far closer than 0.6m when in TTL mode with the flash attached to the camera.

I have proven this by positioning the rig at the closest possible distance to the subject (flash front 0.24m away, lens MFD 0.314m) using aperture/ISO as per the charts and the TTL exposure's fine and in manual flash there's ample power to blow the scene right out.

[george013]

I agree Dave, in fact rather than a "rule of thumb" I would say this 0.6m that's used in the spec charts for TTL capabilities is "a figure totally not applicable to close/macro work", and this thread was about using a macro lens and flash.

In addition I would go as far to say that the info in my flash manual that states ...........


is rubbish, because sufficient illumination giving good exposure is obtained at distances far closer than 0.6m when in TTL mode with the flash attached to the camera.

I have proven this by positioning the rig at the closest possible distance to the subject (flash front 0.24mm away, lens MFD 0.314m) using aperture/ISO as per the charts and the TTL exposure's fine and in manual flash there's ample power to blow the scene right out.

Graham,



Add a lens in this drawing. Let's say this 85mm and a m=1. The focus distance will be 340mm. Do also add the thickness of that lens. The lens sizes are 73 diameter and 98mm length. With a m=1 the focal distance would be 340mm, distance sensor subject.
Go down somewhere halfway the 600 and draw a subject equal in size of the sensor. It's out of the flash beam.


@Dave,
I already noticed my fault.

George

[Stagecoach]

Graham,

Add a lens in this drawing. Let's say this 85mm and a m=1. The focus distance will be 340mm. Do also add the thickness of that lens. The lens sizes are 73 diameter and 98mm length. With a m=1 the focal distance would be 340mm, distance sensor subject.
Go down somewhere halfway the 600 and draw a subject equal in size of the sensor. It's out of the flash beam.

George

No point to do that George, as I have already covered the practical result in post No 31. The lens does not block the light and at any zoom position of the flash ample light reaches the subject. We also have absolutely no idea exactly what that beam is doing, it does not suddenly cut off at its edges.

I have proven this by positioning the rig at the closest possible distance to the subject (flash front 0.24mm away, lens MFD 0.314m) using aperture/ISO as per the charts and the TTL exposure's fine and in manual flash there's ample power to blow the scene right out.

I have posted the results in another thread regarding macro lighting quality. What was significant was that the exposure achieved in TTL at these very close distances was just about equal to the exposure achieved using no flash in auto aperture.

To me this shows that we should not believe everything we read or suspect will happen, and also how accurate TTL is in such unfavourable positioning.

Remembering of course that I did not have the lens hood on

[Artshot]

Thanks for all the replies. Sorry I didn't join in any arguments but I hardly know enough to turn the flash on.
My previous post "first flash" was quite clear that I wanted a flash that had auto aperture mode, as that would work with the camera/lens I have. The SB-910 has 6 flash modes, and I can use 5 of them.
Auto aperture mode use's camera info for focal length and aperture. It then measures reflected light to set power.

I never had any expectations of using TTL.

[Artshot]

Just to try and clarify things, the first post was about information on buying a flash, and I bought the SB-910.

This thread I was asking about what lens people were using for macro. I have seen macro lenses from 50mm to 200mm, which changes the working distance from a few inches to over 2 feet.

I was guessing that this large range in working distance would effect how the flash was used, and would be useful in making a flash bracket, and also in deciding which lens I should use.

Time to post a new thread.

[Stagecoach]

Thanks for all the replies. Sorry I didn't join in any arguments but I hardly know enough to turn the flash on.
My previous post "first flash" was quite clear that I wanted a flash that had auto aperture mode, as that would work with the camera/lens I have. The SB-910 has 6 flash modes, and I can use 5 of them.
Auto aperture mode use's camera info for focal length and aperture. It then measures reflected light to set power.

I never had any expectations of using TTL.

Art,

For me, the thread progressed due to the mention of "flash minimum working distance" in the original post which is a figure given in flash specifications. This "flash minimum working distance" is given in both standard TTL modes and the modes where the flash reads the 'preflash'.

What then progressed was discussion on what 'technically' determines this figure that is given, of which there seems no clear answer although the manufacturers must know the reasoning for it.