first & second focal plane

[foxshot]

Without sounding totaly stupid i often read about first & second focal plane in scopes ,what is the difference between the two & has one got the advantage over the other

[happyhunter]

hi, iam new to the forum but have been reading alot of the topics on here for some time now which are all very intresting. the difference between the two is that in a ffp varibal scope as the mag is changed so does the size of the reticule so you can range targets with it on any mag and the outcome will be the same the downside being that the higher the mag the more the reticule covers the target, on a sfp variable scope the reticule stay the same no matter the mag so the reticule dosent cover the taget but you can only range on a set mag (usually x10 or full mag) unless you have a great memory for what every mag measures at a certain range with that reticule. i use a nf nxs 5.5-22x56 with the np-2dd reticule for foxing and longer range varminting, at full mag the the top of the post from the centre dot at 100y is 2moa, at x11 mag its 4moa at 100yards and at x5.5 mag its 8moa at 100y. fixed power scopes are not affected so i usually leave the nxs on x8 for foxing under the lamp for a good field of view and know how much the fox should fill the reticule out to 250y so allowing for slight holdover, any futher than this and the mag is turned up and the range finder comes out and the scope is dialled in. hope this is of some help. all the best to every for 2012 and straight shooting.

[foxshot]

cheers hh has cleard it up indeed so sounds like the sfp would be the best

[brown dog]

Have a read of this:

 

http://www.premierreticles.com/pdfs/2009-HowToFFP-SFP.pdf

[James19306]

hi, iam new to the forum but have been reading alot of the topics on here for some time now which are all very intresting. the difference between the two is that in a ffp varibal scope as the mag is changed so does the size of the reticule so you can range targets with it on any mag and the outcome will be the same the downside being that the higher the mag the more the reticule covers the target, on a sfp variable scope the reticule stay the same no matter the mag so the reticule dosent cover the taget but you can only range on a set mag (usually x10 or full mag) unless you have a great memory for what every mag measures at a certain range with that reticule. i use a nf nxs 5.5-22x56 with the np-2dd reticule for foxing and longer range varminting, at full mag the the top of the post from the centre dot at 100y is 2moa, at x11 mag its 4moa at 100yards and at x5.5 mag its 8moa at 100y. fixed power scopes are not affected so i usually leave the nxs on x8 for foxing under the lamp for a good field of view and know how much the fox should fill the reticule out to 250y so allowing for slight holdover, any futher than this and the mag is turned up and the range finder comes out and the scope is dialled in. hope this is of some help. all the best to every for 2012 and straight shooting.

 

Although well explained about the difference between the two you are incorrect about the fact the reticle on a ffp scope gets bigger and covers more of the target at higher mag. In first focal plane scopes the reticle will appear to get bigger but remains relative to the target size, as you increase the mag the reticle will appear thicker but dont forget the image also seems to be getting bigger at the same time. Also ffp scopes will never change poi with mag change as with some lesser quality sfp scopes. http:// www.schmidtbender.com/faqs.shtml Interesting bit at the attached link about twilight factor, everyone would consider the 56mm lens to be brighter but in the case shown the 50mm is brighter

[brown dog]

Interesting bit at the attached link about twilight factor, everyone would consider the 56mm lens to be brighter but in the case shown the 50mm is brighter

 

Not an optics expert (be interested to hear the view of Strangely Brown (who is!)), but I think twilight factor is a bit of a weird one; if that 50mm scope could be dialled up to 50 power, its twilight factor would be 50; so more than double both the egs; so WOW! ...but we all know looking through a 50x50 at dusk would be pointless; I think twilight factor puts too much emphasis on the effect of magnification.

[ds1]

"The twilight factor can be a useful measure of the capability to see details in low light conditions, but is dependence of adequate brightness (exit pupil) to have any value. What is adequate brightness is dependence of the outer light conditions.

 

The big disadvantage of the twilight factor is that it increases without limit with the magnification, without taking in consider the exit pupil will decrease without limit.

For example, a 10x50 and even a 12x50 show more details even under such low light conditions than a 7x50, but if the light conditions become bad enough the 7x50 will win, because no magnification helps if the brightness is too low. (Magnified darkness = darkness)".

[Strangely Brown]

Not an optics expert (be interested to hear the view of Strangely Brown (who is!).

 

Me'thinks you have too much faith in me!

 

"The twilight factor can be a useful measure of the capability to see details in low light conditions, but is dependence of adequate brightness (exit pupil) to have any value. What is adequate brightness is dependence of the outer light conditions.

 

The big disadvantage of the twilight factor is that it increases without limit with the magnification, without taking in consider the exit pupil will decrease without limit.

For example, a 10x50 and even a 12x50 show more details even under such low light conditions than a 7x50, but if the light conditions become bad enough the 7x50 will win, because no magnification helps if the brightness is too low. (Magnified darkness = darkness)".

 

Completely agree with this!

 

Many years ago (40+) Zeiss made great play with the "Twlight Factor" in their advertising but all this did was show confusion in the market place, especially amongest stalkers who favoured 7x42 or 8x56 at that time.

 

The consumer in those days was confonted with a Ziess 10x42B Dialyt which had a bigger twilght factor, but in reality was not fit for the purpose of stalking in low light where the ability to see movement was lost with the higher magnification.

The above generally refers to stalking in the home counties rather than Scotland, where (as a retailer) more 10x glasses and draw scopes are sold as opposed to 8x binoculars.